Trunk injected fungicide treatments evaluated for management of Bot canker fungus Diplodia corticola on northern red oak trees, Quercus rubra.
With the challenges that negatively impact tree-based agriculture, landscapes and forests, such as climate change, plant pathogen and insect range expansion, invasive species and limited new pesticides, it is important to introduce new and effective tree protection options. In the last 20 years, pathogens that invade wood i.e. vascular tissues of trees causing wilt, yellowing, premature defoliation, cankers and tree death, have been on the rise. Diplodia corticola causes Bot canker of oak species which can kill trees diminishing the valuable ecological services they provide and reducing profits from wood and cork production. Since this and similar pathogens have difficult biologies because they reside in wood and cause severe internal damage and tree death, their management is difficult or inefficient with classical pesticide application methods that cannot reach and distribute the active ingredient in vascular wood tissues. As practical management options for this and other vascular tissue pathogens of trees are limited, we evaluated efficacy of several trunk injected fungicides in control of D. corticola and compared it with the efficacy of trunk injection of similar compounds for control of Venturia inaequalis and Erwinia amylovora, as two well-studied apple tree pathogens with different or partially similar lifestyles to D. corticola, respectively.
- trunk injection of pesticides
- tree disease management
- Diplodia corticola
- Venturia inaequalis
- Erwinia amylovora
Agricultural, urban, and natural tree stands have been the focus of extensive plant pathogen diagnostic and disease management research in recent decades [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16] which recorded an increase in the number of new fungal and bacterial pathogens and their detrimental impact on agroecosystems, ecosystems, and the human society. The economic effects of these pathogens are reflected in lost fresh fruit produce [17, 18, 19], reduced yields and quality of fruit or wood and cork products [20, 21], diminished ecological tree services, and death of whole trees, stands, and forest regions or decimation of fruit industries [19, 22].
If left unmanaged, apple scab fungus
The biology of majority of these microorganisms, excluding
Tree injection, often referred to as trunk or stem injection, is a method of target precise delivery or application of pesticides, plant resistance activators and fertilizers into the xylem vascular tissue of a tree with the aim to protect trees from insect pests and pathogens or to provide tree nutrition and/or correction of micronutrient deficiencies. It primarily harnesses the transport capacity of the tree’s vascular system driven by transpiration stream of water in these tissues to translocate and distribute the active compounds into the trunk, branches, canopy and roots where protection or nutrition is needed. Tree injection as a plant protection method is viewed as environmentally safer alternative for pesticide application because it secures significant reduction of non-target exposure of water, soil, air and wildlife to pesticides and fertilizers in landscapes and urban greening areas. The active ingredients are delivered within the tree, thus providing selective exposure to plant pests, with limited negative effect of weather conditions like rain or sun radiation on the injected compound and with creating no immediate pesticide residue losses outside the tree.
Trunk injection relies significantly on tree physiology processes related to water transport, xylem and phloem tissue functions, and the weather conditions that influence these specific plant processes. To achieve delivery of an effective pesticide dose, its distribution and expected management of plant detrimental organism or nutrient deficiency, there are several key factors which should be monitored by an applicator as they influence success of trunk injection for these purposes. Besides the plant pathogen biology, ecology, and epidemiology, several factors play a key role in success of trunk injection efficacy: the time of application in relation to detrimental organism establishment and symptom occurrence , the season and time of the day of application , the chemical properties of pesticide active ingredient and its formulation , the injected volume or dose of a pesticide, and the type of tree injection device or technology. For example, a more effective management of plant disease or insect infestation can be achieved by the preventive injections of pesticides in comparison to the therapeutic pesticide applications after the disease or infestation has already occurred. Tree injection of active compounds is much faster and easier during spring and early summer months in comparison to the late or mid-summer, late fall and winter, because water in the soil is abundant and the green leaf canopy is facilitating intensive transpiration pull and flow of water through the xylem tissue in hardwood trees, starting from the roots and branches to the leaves . The three key properties of injected active ingredient that determine its mobility or binding in xylem of the tree are organic carbon-water partitioning coefficient (ml/g or μg/g) or carbon adsorption coefficient (Ko/c), water solubility, and formulation type. Ko/c expresses the level of adhesion or adsorption of pesticide active ingredient to the carbon rich compounds in certain environments such as soil or xylem and is defined as a ratio of mass of a chemical that is adsorbed in a certain environment per unit mass of organic carbon in that environment per the equilibrium chemical concentration in a solution. Active ingredients that have high Ko/c values will strongly bind to the organic compounds present in soil, sap or xylem and reduce their systemic movement i.e. translocation, accumulation and distribution in the canopy, thus reducing the efficacy in pathogen or pest control. In contrast, the ingredients with low or moderate Ko/c values move and distribute fast after tree injection and distribute well in the canopy, securing good pest or pathogen control. Pesticide formulation is a form of a pesticide active ingredient ready for use or which quite often requires dilution in water prior to application. Formulation is made by adding different inactive ingredients with the aim to improve the properties of an active ingredient such as solubility, surface adhesion, distribution, effectiveness, shelf life, stability, handling or application (e.g. solvents, emulsifiers, surfactants and other adjuvants). Formulation of a pesticide or a fertilizer determines the properties and residue stability of an active ingredient and can modulate its mobility in xylem of phloem after tree injection for pest or plant management [12, 42]. Finally, trunk injection devices can loosely be divided into the ones using drill- or needle-based technology . In case of the first one, access to xylem for pesticide delivery device is enabled by drilling into the trunk or root flare wood, removing a small part of the wood, and sealing of the opened injection port with an inserted plastic plug or not (plug contains an injection valve with a one-way silicone septum) . For the injection application to be faster and hence economical in urban tree care, this system uses compressed air or hydraulic pressure to force-inject the pesticide solution into the wood. The second technology uses a knife-like or a flat, screwdriver-like needle with a lenticular profile, which is inserted into the wood by a hammer thus separating the wood fibers and creating a crevice while the delivery of a pesticide solution is conducted through the needle and infused into the wood . This system can use force of hydraulic or compressed air pressure to deliver the pesticide solution into the xylem or is solely relying on the Venturi effect (vacuum) created by a transpiration stream in xylem to infuse the pesticide solution into the wood [45, 46, 47]. This injection technology requires longer time for injection solution delivery, especially when transpiration is limited, and thus is often less economical in urban tree care.
Tree injection was initially developed for pesticide and fertilizer application on large size trees in proximity of urban areas where ground- and air-spray applications are impractical due to substantial pesticide losses through drift, lack of proper canopy coverage, or are prohibited due to possible human and domestic animal exposure. The second driver for development of tree injection and its more frequent use in recent decades has been the destructive nature and an increasing need for effective management options for invasive tree pathogens like
Due to a demonstrated ability of single trunk injection to increase the efficacy of injected pesticides over multiple years, a possibility to reduce the of number of topical spray applications [10, 12, 48] and a rising incidence of woody plant pathogens and insect pests in the environment [31, 33, 49, 50, 51], this approach has recently been investigated in agriculture where topical pesticide applications for plant food production is intensive. The most investigated tree fruit crops and their pathogens and insect pests are citrus (e.g.
While trunk injection for pesticide delivery is a relatively new technology investigated in tree-based agriculture for managing diseases like citrus greening [14, 15, 16, 66, 67, 68] or fire blight [11, 44], research in agricultural engineering will first need to design or invent an application system/s that allow scalability, i.e. achieving simultaneous trunk injection of large number of trees in a short period of time. Besides this end goal many other key questions arising from research outlined above will need to be addressed through experimental work before tree injection is used in agriculture, even in limited fashion. The first steps are providing enough evidence i.e. providing proof of concept that injected pesticides are effective in tree pathogen and insect management and that injected materials have minimal negative effect on fresh fruit consumer and beneficial orchard fauna. Because effective management options for Bot canker and decline of different
2. Trunk injection delivery of pesticides for management of three important plant pathogens in continental climate
2.1 Biology of
Diplodia corticola, Venturia inaequalisand Erwinia amylovora
2.1.1 Diplodia corticola
In the binomial nomenclature of fungi, Bot canker pathogen
Majority of evidence indicates that
2.1.2 Venturia inaequalis
The sexual stage of an ascomycete fungus
Depending on the substrate it colonizes over the year, the life cycle of apple scab fungus has the saprophytic and the parasitic phase in its development. The saprophytic phase starts with apple leaf drop in autumn.
During the time after spores of apple scab fungus land on the susceptible plant surface, while they germinate, and up to 72 h after they penetrate below the cuticle on green tissue, they are vulnerable to spray-applied contact and systemic fungicides, respectively. However, once the infection is established by formation of mycelium under the cuticle, almost all fungicides applied to green plant surfaces have no efficacy in eradicating these infections and eventually lesions with conidia, or their effect is minimal. Furthermore, continued post-infection scab management with spray applications of fungicides that aim to prevent new infections on green tissues is complicated by large populations of conidia, which if exposed to fungicides with specific modes of action might increases the potential for fungicide resistance selection in this devastating pathogen. Because of the specific lifestyle of
2.1.3 Erwinia amylovora
Fire blight is caused by a Gram-negative bacterium
During the incubation, i.e. usually sometime around 10 – 14 days before the first conspicuous blossom or shoot blight symptoms are visible, small white, amber or orange droplets of bacterial ooze can emerge and drip from the infected green tissues (flower pedicels, floral cup, sepals, immature fruit and shoots). With more wetting events and insect activity, ooze can spread to new flowers and actively growing shoots across the whole orchard. Since blossom and shoot blight symptoms are not yet visible, this dissemination of ooze allows secondary infections and can propel a fire blight outbreak into an epidemic, especially if the antibiotic spray application/s were not conducted during bloom. Once incubation is over, blossom blight is visible as dead, black or brown flower clusters with more droplets of bacterial ooze developing if weather conditions are humid. Shoot blight and immature fruit infections are visible as black or brown “flags” or “strikes” and brown to black shriveled fruitlets, respectively. Blighted shoot tips often bend in the typical shape of Shepherd’s crook. Fire blight cankers on branches, trunk and rootstock are formed by pathogen’s progress via xylem or the cortical parenchyma from the established infections on flowers, shoots and suckers, into the wood bark tissues.
2.2 Materials and methods
2.2.1 Trunk injection of pesticides for Diplodia corticola management
To test the effect of injected fungicides and activators of plant systemic acquired resistance (SAR)  for reduction of Bot canker caused by
One injection point i.e. port per trunk of each potted tree, positioned ca. 5–7 cm above the ground level, was created by drilling 7–10 mm into the xylem tissue with a 4.3 mm diameter drill attached to a cordless drill. To inject the protective liquid solutions listed in Table 1, we used a Stinger needle for plugless trunk injection assembled on an individual feed line attached to the Tree IV air/hydraulic microinjection system, which operated at 60 psi air pressure (Arborjet Inc., Woburn, MA). The Stinger needles are used for injection of trees when trunk injection ports of large diameter (9.5 mm) are of concern or should be avoided and for injection of trunks with small diameters. The diameter of injection port for inserting a Stinger needle is smaller and does not require sealing with an Arborplug. In year one, the injected potted oak trees had trunk diameter at 5 cm height averaging 1.3 cm and ranging from 1 to 2.1 cm. In year two, a new set of injected trees had the diameter at 5 cm height averaging 1.5 cm and ranging from 1.1 to 2.2 cm. Trunk injection were conducted on 12 June in year one and on 16 August in year two.
Trees were inoculated with
Statistical analysis was done with MIXED procedure in SAS Studio software (SAS Institute Inc. 2017, Cary, NC) using the xylem necrosis areas (cm2). If the fungicide effect was found to be statistically significant (
2.2.2 Trunk injection of pesticides for Venturia inaequalis management
With the goal to optimize timing and number of fungicide injections for management of apple scab fungus
On each trunk injection date with fungicides listed in Table 2, a separate set of four cardinally-oriented trunk injection ports per each tree of ‘Mac Spur’ was created by drilling 25 mm into the xylem with a 9.5 mm diameter drill bit attached to a cordless drill. The first set of four injection ports was positioned ca. 25 cm above the ground level. The subsequent sets of four injection port were positioned ca. 5 cm above and between the lower four-port sets. Every port was sealed with Arborplug no. 4 (Arborjet Inc., Woburn, MA) positioned just below the bark level to allow port closure with cambium callus . To inject the fungicides, we used the Quik-jet microinjection system (Arborjet Inc.) operating at hand-generated hydraulic pressure to deliver low volumes of liquid for injection, thus allowing faster application times, and the Tree IV air/hydraulic microinjection system (Arborjet Inc.) operating at up to 60 psi of air pressure to deliver large solution volumes of liquid for injection (≥600 ml). In the experiment 1 (Table 2), we injected all the treatments listed for 15 October in year one with Quik-jet (Table 2). On 11 April in year two, we injected propiconazole using the Viper air/hydraulic microinjection system set at 90 psi air pressure (Arborjet Inc.). At the later dates, we injected Alamo using the Tree IV and Phosphojet using the Quik-jet. In the experiment 2, we injected Phosphojet with Quik-jet and cyprodinil + difenoconazole with Tree IV. The needle/s of each of the used injection devices was inserted through the Arborplugs allowing the total liquid volume per tree, at one injection time, to be divided and delivered equally among the four ports.
All the experiments were conducted under naturally high infection pressure during the primary season of
2.2.3 Trunk injection of pesticides for Erwinia amylovora management
126.96.36.199 Treatments for reducing blossom and shoot blight incidence
To test the effect of injected bactericides and activators of plant systemic acquired resistance (SAR) for blossom and shoot blight incidence reduction, the orchard experiments were conducted over 2 years (Table 3). The early spring injections in year one (26 March) were conducted with Viper air/hydraulic micro-injection system® at under 110 psi of air pressure and late spring injections (23 April) were done with Tree IV® air/hydraulic micro-injection system, at 60 psi air pressure (Arborjet Inc., Woburn, MA). In the year two, trunk injections on 1 and 22 May were applied using Tree IV® air/hydraulic micro-injection system at 60 psi of air pressure. The injection needles of these devices were inserted through the one-way valve silicone septum in the Arborplugs® which allowed delivery of protective solutions into he drilled injection ports. In each injection, the total injected volume per tree was divided equally among the four ports (Table 3). Four injection ports per each apple per tree, positioned ca. 10–15 cm above the ground level, were cardinally oriented and created by drilling 25 mm into the xylem tissue using a 9.5 mm diameter drill bit attached to a cordless drill. Each port was sealed with Arborplug® no. 4, by pushing the plug with a specialized screwdriver-like tapper hit with a hammer (Arborjet Inc., Woburn, MA, USA). The plug was positioned just below the bark level to allow port closure with cambium callus.
In the year one, we used 14-year-old ‘Gala’ apple trees which were trunk-injected using the compounds and dosages listed in Table 3. Injections were performed at the tight cluster growth stage in apples (26 March), or 21 days before 80% bloom, and at petal fall growth stage (23 April). In the year two, experiments were conducted on a new set of 21-year-old ‘Gala’ apple trees, injected with the same doses in Table 3. Injections were applied at early tight cluster growth stage (1 May) or 13 days before 80% bloom and at petal fall (22 May). The treatment Actigard 1 was injected only on the first date in both years (Table 3). Each dose in every treatment, except the Phosphojet, was diluted and injected with 520 ml of water per tree. The doses per tree were chosen according to the four rules: (1) the dose was equivalent to the US EPA pesticide label rate for a maximum amount per 0.405 ha with 250 planted apple trees; (2) the dose was one half of the maximum US EPA label rate allowed per one season; (3) the dose was equal to a rate delivered in one spray application treatment per 0.405 ha with 250 apple trees; or (4) the dose was selected based on previous research with trunk injection of similar pesticides . Trees injected with water and the non-injected non-inoculated trees served as negative controls for efficacy comparisons. In year one, each treatment was replicated on four trees arranged in a randomized complete block design, where blocking controlled the variable crown tree sizes (large, medium, medium-small, and small) . In the year two, we used the same number of replicate trees per treatment but arranged in a completely randomized design (CRD).
In year one, on 16 April at 80% bloom, apple flowers of all experimental trees were inoculated with a suspension of
We analyzed the data with MIXED procedure in SAS 9.3 (SAS Institute, 2012). The main effect of treatments on blossom and shoot blight incidence were analyzed using
188.8.131.52 Treatments for reducing shoot blight severity
To test the reduction of shoot blight severity with bactericide oxytetracycline hydrochloride (Arborbiotic, MFG Scientific Inc., EPA Reg. No 88482-1; Arbor-OTC® Injectable Tree Antibiotic, Arborjet Inc., Reg No. 74578-7), apple trunk injections were performed in a similar fashion described above, but by using a Quik-jet® micro-injection system instead (Arborjet Inc., Woburn, MA, USA). This device relies solely on hand-generated hydraulic pressure to inject the necessary pesticide solution volume in each port. The injection ports were created and sealed with Arborplugs (Arborjet Inc.) in the same way as described above and injected volume per tree was divided equally among the four ports. The experiments were conducted in 2 years. In year one, at petal fall growth stage (23 April) mature 12-year-old apple trees of cv. ‘Jonathan’ were trunk-injected with Arborbiotic using dose in Table 3 diluted at 10% in water. The total dose per tree was calculated based on the unique trunk diameters at 30 cm height using the EPA label instructions. In year two, the same apple trees injected in year one were re-injected at petal fall (22 May) using the same dose in Table 3 delivered via a fresh set of drilled injection ports above the previous year’s set of injection ports. In both years, Arborbiotic treatment as well as water control were replicated on four trees arranged in a CRD.
A total of 10 terminal shoots per each tree were inoculated on 7 May in year one and on 30 May in year two. We used a previously reported inoculation method . In brief, the upper third of leaf blade of the second or the third youngest leaf on each shoot tip was cut perpendicular to the leaf midvein with scissors dipped in
We analyzed the data using MIXED procedure in SAS 9.3 (SAS Institute, 2012). If the main effect of treatment on shoot blight severity was found significant (
3.1 Trunk injection of pesticides for
All the three fungicides trunk-injected preventively provided significant reduction of Bot canker caused by
3.2 Trunk injection of pesticides for
In the experiment 1, fungicides injected four times in total, once in fall and then three additional times in spring, during the primary scab infection period, provided significant reduction of apple scab incidence on spur and shoot leaves (Figure 2A). On spur leaves, the best scab reduction of 45.5% was achieved with injected Phosphojet high, but this control was not better in comparison to 78.6% in spray standard applied in spring during the primary scab season (Figure 2A). In contrast, control with injected Phosphojet high on shoots outperformed the spray standard with 73.6 vs. 62.9% in scab reduction (Figure 2A). Similarly, Alamo performed better on shoot leaved than on spur leaves (Figure 2A).
In the experiment 2, fungicides injected 1–2 times in total, across or within two seasons of fall and spring, revealed that the injected Inspire Super treatments largely did not significantly reduce disease incidence on spur and shoot leaves when compared to the water control. In contrast, all the injected Phosphojet treatments and Agrifos sprays did. Comparisons among these treatments clearly demonstrated that on all the three rated apple organs (Figure 2B), Phosphojet trunk injections provided statistically better apple scab reduction i.e. control in comparison to all the Inspire Super trunk injections. On spur leaves, two Phosphojet trunk injections, fall plus spring, was the best treatment among injections by providing 46.3% control which was similar to the Inspire Super sprays (Figure 2B). On shoot leaves, two Phosphojet trunk injections both done in spring, provided the best scab control of 66.5% similar to nine sprays of Agrifos (Figure 2B). On fruit, scab control was the best in Phosphojet trunk injection done once or twice in spring, and in fall plus spring: 62.8, 69.7 and 64.6%, significantly outperforming both the Agrifos and the Inspire Super sprays (Figure 2B).
3.3 Trunk injection of pesticides for
3.3.1 Treatments for reducing blossom and shoot blight incidence
In both year one and year two, all the trunk-injected bactericides (Agrimycin) and SAR-activators (Actigard, Phosphojet) provided significant reduction of blossom blight incidence in comparison to the water control (Figure 3). In year one, which had low disease pressure (Figure 3A), there was no significant difference among all the treatments in disease reduction i.e. control (37.9–61.1%). In year two, with high infection pressure, Agrimycin was the best providing 28.9% blossom blight control (Figure 3B). Averaged across both years, Agrimycin and then Phosphojet were the best treatments with 45 and 40.5% achieved control, respectively (Figure 3).
In year one, none of the trunk-injected products provided significant reduction of shoot blight incidence in comparison to the water control, hence did not differ among each other (Figure 4A). In year two, under high disease pressure, all the injected products significantly reduced shoot blight incidence for 23.4–36.5% in comparison to the water control, but when compared they did not significantly differ between each other (Figure 4B). If averaged across both years, Agrimycin and then Phosphojet achieved the best control of 53.5 and 42.8%, respectively (Figure 4).
We present the first data on management of
The organic carbon-water partitioning coefficient (Ko/c) for thiabendazole is moderate to high and ranges from 1104 to 4680 ml/g, while water solubility is 50 mg/L at pH 7 and 38 mg/ml at pH 2 . These parameters indicate on low to no mobility of thianbendazole in xylem as a carbon rich environment. The Ko/c of propiconazole is 1086–1817 ml/g which is moderate to high [119, 120] and water solubility is low, 100–150 mg/L . This could have contributed to slow and reduced uniformity in distribution of injected fungicides in xylem. However, both Arbotect and Propizol are fungicides formulated for trunk injection on hardwood trees and if properly diluted and delivered preventively they can accumulate sufficiently to secure the internal control of specific plant diseases (e.g. Dutch elm disease caused by
In the future studies, we predict that the efficacy of preventive fungicide applications against
We evaluated the similar fungicides on apple,
The efficacy against this subcuticular pathogen that infects just below the waxy layer on leaves and fruit, clearly depended on the apple canopy organ and the time/s of fungicide injection/s. Namely, on spurs which hold much fewer leaves in total in comparison to the shoots, the best leaf scab incidence reduction was 45.5 and 46.3%. In contrast, scab reduction on shoot leaves with Phosphojet reached 66.5 and 73.6%. On apple fruit, scab reduction reached up to 62.8, 64.6 and 69.7%. These efficacy patterns clearly demonstrate the differential influences of the tree’s yearly and organ-specific physiology, the properties of injected compound, and the injection timing on the accumulation of fungicides in the canopy. Since the major water transport in xylem, occurs in spring, at least one to two injections of phosphites in early spring gave a good disease control, depending on the canopy organ. The best scab control with injected phosphites was achieved on the shoot leaves, followed by apple fruit, and then on the spur leaves. The injected phosphites probably accumulated more in the shoot leaves than in the spur leaves and they accumulate more in fruit than in spur leaves. This can be explained by the variable rates of transpiration from these organs, which influences the speed and abundance of fungicide accumulation after trunk injection. The total leaf area on shoots is larger in comparison to spurs. The fewer leaves on spurs, which are first to develop in spring and early reach their full size, have fewer total number of stomata on them in comparison to more numerous shoot leaves. Additionally, from petal fall up until terminal bud set, shoots keep growing and developing more leaves on the tips. Hence, apple shoots hold the higher number of stomata in total, thus allowing much higher transpiration intensity, abundant accumulation of injected fungicides and thus scab control. Similarly, apple scab control was lower on fruit than on shoots which could be explained by the fact that apple fruit hold 10- to 100-fold lower frequency of stomata on their epidermis in comparison to the apple leaves .
The chemical properties of different active ingredients impact their distribution and accumulation in the canopy. For example, potassium phosphites have higher water solubility of 500 g/L in comparison to propiconazole and difenoconazole which have low to very low water solubilities of 100–150 mg/L and 13 mg/L, respectively [121, 123]. Potassium phosphites have low organic carbon-water partitioning coefficient (Ko/c) from 228 to 587 ml/g in comparison to moderate to high of propiconazole, 1086–1817 ml/g, and of difenoconazole, 3870–11,202 ml/g, respectively [119, 120]. This difference likely allowed phosphites to move faster in xylem  and accumulate more in leaves and fruit than the other injected fungicides. At the same time, propiconazole and difenoconazole were probably bound to the organic phase of xylem symplast and apoplast, thus lowering their accumulation in leaves and fruit and reducing their effect on scab incidence . This is often referred to as a reservoir effect and Ko/c as is an important property of a pesticide that can explains its limited or abundant accumulation in the canopy [65, 125]. Besides the Ko/c and water solubility, the inactive components of the Inspire Super pesticide formulation we injected (stickers, emulsifiers, surfactants, etc.) could reduce the abundant accumulation of difenoconazole and a better scab control. Fungicides have to be formulated for injection to secure their upward translocation in xylem and often diluted prior to trunk injection to reduce the impact of Ko/c effect. Once the high solubility, low Ko/c and injectable formulation are possible for one active ingredient, a rapid and desired control effect on plant pathogen or insect pest can be expected [42, 45, 126].
The reduction of apple scab and our prior work on analyzing the residues of injected pesticides on apple leaves and fruit [12, 61] indicates that accumulation of trunk-injected fungicides in the wood and canopy is a time-demanding process chiefly shaped by the tree physiology and tissue resistance points [127, 128]. Trunk injection is an opposite process to the immediate deposition of fungicide solution on the tree canopy by foliar spray applications. However, even though the injected dose per tree of phosphites in Phosphojet was 1.6–2 times higher than in the Agrifos sprays, the fact that just two injections secured better control of scab on fruit and spur leaves in comparison to nine Agrifos sprays demonstrated better persistence of injected Phosphojet. This shows that trunk injection is a superior delivery method for phosphites as it enhances their activity for 1–2 growing seasons .
The fire blight bacterium
The best control i.e. reduction of blossom blight incidence across both trial years was achieved with two trunk injections of Agrimycin (45%) and of Phosphojet (40.5%). However, under high and low infection pressures in the two trial years, the levels of control with these materials (28.9, 61.1%, 25.1, 55.9) were far from comparable to 92–99% control often achieved and expected with preventive flower spray application of Agrimycin and Kasugamycin in commercial apple orchards [129, 130]. In the case of injected Phosphojet and Actigard, the achieved blossom blight reduction probably originated from an SAR effect triggered in the nearby spur leaves by these compounds, as the SAR effect in flowers was inconsistent . SAR is a defense plant response which is activated after localized plant exposure to a pathogen or after a spray applications of a synthetic or natural compound, known as an SAR-inducer or activator . Our 1–2 trunk injections of Actigard reduced blossom blight incidence for only 19–42%, indicating that this delivery method cannot not improve the SAR-effect of Actigard on flowers to combat blossom blight successfully. Namely, different sources report from 3 to 91% of blossom blight control with foliar sprays of Actigard on other apple cultivars [132, 133, 134].
Vegetative flowers parts in
Even though reduction of shoot blight incidence was not statistically significant in year one, which was characterized with low infection pressure, it indicated that trunk-injected Agrimycin and Phosphojet might have potential to perform better than Actigard treatments. However, in year two, under the heavy infection pressure, this was not the case as all the injected treatments were similar. Overall, it seems that the reduction of shoot blight incidence with injected Agrimycin and Phosphojet across both years of 53.5 and 42.8%, was slightly better than the reduction of blossom blight incidence with the same materials of 45 and 40.5%, respectively. Shoots obviously have much higher green tissue area and transpiration rate in comparison to the flowers. Shoots likely accumulate higher amounts of trunk injected compounds in comparison to the green flower parts, which allowed slightly better disease reduction early after injection. Still, the shoot blight incidence reduction was far from the expected control with spray applied antibiotics in commercial apple orchards. In a trial with trunk injection of Arborfos (45.8% mono- and di-potassium salts of phosphorous acid, Mauget Inc., Arcadia, CA, USA), shoot blight was reduced for 67% on inoculated ‘Paulared’ apple trees . The same dose per tree which we delivered in two injections of Phosphojet, achieved shoot blight incidence reduction of 23.4–62.1%. Since we have split the dose delivery temporally, this weakened shoot blight incidence reduction by Phosphojet and probably by Actigard too. In shoot inoculated trials multiple Actigard sprays achieved shoot blight reduction between 2.8 and 50.7% [135, 136] while by trunk injection we achieved only 1.7–30.9% of shoot blight reduction. Hence, the two-time trunk injection does not improve shoot blight reduction by Actigard.
The reduction of shoot blight severity with Arborbiotic (MFG Scientific Inc., USA) was excellent and reached up to 82%. Such an effect with oxytetracycline hydrochloride demonstrates that this active ingredient is readily soluble in water and that the formulation we used is designed for trunk injection. Our results indicated that the trunk injected Arborbiotic limits i.e. stops systemic spread of
Our results on management of three different pathogens with partially similar or different biologies, where
In the biology i.e. life cycle of
In the case of
Finally, there is the case of a complex biology of
The work on
The work on
The fire blight work was funded by USDA-NIFA Pest Management Alternatives grant MICL05066 in 2012 and continuation grant MICL07748 in 2013–2014 to JCW for project “Trunk Injection: A Discriminating Delivery System for Tree Fruit IPM”. The work with evaluating Arborbiotic in reduction of fire blight severity was funded by Arborjet Inc., Woburn, MA, USA.
We thank Joseph J. Doccola, director of research and development at Arborjet Inc. and John J. Aiken, regional technical manager, for donating injection equipment and chemicals used in apple scab and fire blight experiments. For assistance in conducting or facilitating experiments, we thank research staff of at Michigan State University’s Trevor Nichols Research Center (TNRC) in Fennville, MI, Anthony VanWoerkom, Gail Ehret, Jerri Gillett, Jason Seward, Laura Lamb and Kyle Coffindaffer. We acknowledge Dr. Annemiek Schilder, Dr. Brad Day, Dr. Jianjun Hao, Dr. Jim Miller and Dr. Randolph Beaudry for generously sharing their laboratory resources at Michigan State University in support of this research.
Conflict of interest
All authors declare that the research was conducted without any commercial or financial relationships that could be interpreted as a potential conflict of interest.