Control of Major Diseases in Horticulture

Emma W. Gachomo1,2, Jose C. Jimenez-Lopez3, Adechola P. P. Kayode4, Lamine Baba-Moussa5 and Simeon O. Kotchoni1,2 1Department of Biology, Rutgers University, Camden, 2Center for Computational and Integrative Biology, Rutgers University, Camden, 3Department of Biochemistry, Cell and Molecular Biology of Plant, Estacion Experimental del Zaidin, Consejo Superior de Investigaciones Cientificas, Granada, 4Departement de Nutrition et Sciences Alimentaires, Faculte des Sciences Agronomiques, Universite d’Abomey-Calavi, Cotonou, 5Laboratoire de Biologie et de Typage Moleculaire en Microbiologie, Faculte des Sciences et Techniques, Universite d’Abomey-Calavi, Cotonou, 1,2USA 3Spain 4,5Benin


Introduction
Cultivation of fruits, vegetables, flowers, and/or ornamental plants is one of the economically important sectors of crop production. However cultivation of these plants plagued with wide range of most destructive, reoccurring and widespread stress factors both biotic (diseases) and abiotic (environmental) (Gachomo et al., 2009). Plant diseases are caused by many different types of organism that include fungi, bacteria, viruses etc. While plant disease may be caused by many factors, majority of them are caused by fungi and fungi like organisms (FLO). Over 8,000 species of fungi and FLOs have been reported to cause disease (Ellis et al., 2008). Disease caused by fungi and FLOs have been recorded to cause damage that has had far reaching effects on human population as a result of the suffering that follow crop loss (Gachomo, 2005) e.g. the great famine of 1845-1852 that occurred in Ireland as a result of potato crop failure due to potato blight infection. During this famine about a million people died of hunger and another million emigrated out of Ireland. Control of plant disease is both expensive and time consuming and represents a large segment of the pesticide market (Gachomo et al., 2009;Killian & Steiner, 2003), despite continuous efforts to control their detrimental effect on crop production. For example cost of control and damage by Phytopthora infestans (that causes late blight of potatoes) in Europe is estimated at more than €1,000,000,000 ($1,400,000,000) per year (Haverkort et al., 2008). The typical symptoms of these fungal induced diseases cover all color spectra (from white to black) and form different types of symptoms both internal and external, microscopic and macroscopic, local and systemic which appear (but are not limited to) as leaf spots, galls, lesions, blights and cankers incase of local symptoms or as wilting, yellowing, and dwarfing incase of systemic symptoms because they involve most or all of the plant (Gachomo et al., 2006;Gachomo & Kotchoni 2007;Gachomo et al., 2010). Common fungal diseases include leaf spots, cankers, powdery mildews, downey mildews, smuts, blights, damping off, In this chapter, we give a detail overview of disease symptoms, their causal agents and fungicide dependent control of selected major fungal diseases in horticulture such as powdery mildew, gray mold, blackspot, late and early blight diseases, and Fusarium wilt disease. Optimum number and timing of fungicide sprays for an efficient disease control in horticulture are here discussed. In addition, we report several aspects of fungicide activity, including the effects on fungal establishment within the host.

Powdery mildew
Powdery mildew as the name suggests initially appears as a white grayish coating on leaves (Figure 1 A). When the disease is severe it produces a white gray coating on stems, petals and buds. This white coating spreads over time to cover the whole leaf (Figure 1 B, C). The affected leaves or petals get distorted, shrivel and may fall to the ground. Several disease cycles occur within the plants' growth season. The disease makes the plants unattractive and repeated disease cycles cause reduced vigor of the plants (Gachomo, 2005). Among the most often affected flowers and shrubs are columbine, dahlia, delphiniums, honeysuckle, ivy, lilac, phlox, privet, snapdragons, lungwort, bee balm, garden phlox, verbena, rose, and zinnias.

Life cycle of powdery mildew causal agents
The powdery mildews are caused by a group of more than 300 related fungal species which are host specific, meaning that a fungal species that attacks one type of plant species might not infect another plant species. Examples of fungi causing powdery mildewinclude Sphaerotheca pannosa that infects roses; Erysiphe cichoracearm that infects Chrysanthemums. The primary inoculum comes from dormant infections that survive the winter. The overwintering structures called cleistothecia produce sexual spores, which infect young leaves and blossoms when the weather is favorable. Mycelia formed in the infected tissues form conidia, which infected other leaves, shoots and buds. Several generations of conidia are formed in the growing season as long as the weather remains favorable. Towards the end of the season when the conditions become unfavorable for disease development, cleistothecia are formed in the infected tissue and will serve as a source of inoculum during the next growing season.

Control and prevention of powdery mildew disease
Spores of the fungal causing disease are spread by wind and water splash; it is therefore crucial to avoid wetting the leaves during watering as one of the most fundamental strategies of powdery mildew control. In addition, avoiding the use of sprinkling irrigation but rather watering at the base of the plants or use of drip irrigation are the general rules in controlling this disease. In order for the disease to develop, the spores require plants wetness for several hours to be able to infect the plants, therefore water the plants preferentially in the morning rather than in the evening so that they have time to dry during the day, therefore reducing time of leaf wetness. Varieties that are susceptible should not be planted under the shade but in areas exposed to the sun because shading does not allow dew to dry quickly. The plants should be well spaced and kept free of weed to increase aeration around the plants and reduce moisture retention around the base of the plant. Plants should be pruned to reduce dense foliage and increase aeration. The fungus overwinters in fallen plant debris, therefore it is very important to clean up all fallen plant materials and prune infected tissues at the end of the season, which will ultimately reduce the source of inoculum for the next season. Up to date, the efficient control of powdery mildew disease relies on fungicide application. Several fungicides have been developed for the management of this disease. In Table 1, we show some of the widely used fungicides in powdery mildew disease control. www.intechopen.com

Life cycle of gray mold causal agents
Gray mold disease is caused by many strains of the fungus Botrytis cinerea. This fungus is not host specific and therefore infects many different types of plant species. Symptoms vary with the plant species attacked. The fungus needs moisture to cause infection and a favorable temperature ranging from 18° C to 25° C.

Control of gray mold disease
To contain the disease all infected tissue should be remove as soon as possible; infected debris should be collected. The plants should be well spaced and weeded to allow good air circulation; sprinkler irrigation and wetting of the foliage should be avoided because this provides the humidity needed for the proliferation of the disease. Application of fungicides www.intechopen.com in combination with the good sanitation measures will ultimately help to contain the disease. It is imperative to apply preventive fungicides as soon as disease is detected. Fungicides with the active ingredients such as chlorothalonil, dichloran, fludioxonil, trifloxystrobin, iprodione, mancozeb, copper sulfate pentahydrate, fenhexamid, azoxystrobin, and thiophanate methyl are registered for Botrytis control and therefore recommended to use in case of gray mold disease. Be sure to rotate applications among chemical classes as fungicide resistant strains of Botrytis have been reported.

Blackspot disease
Blackspot is a disease of roses. It appears as small black spots on the upper surface of the leaves (Figure 3 A), which first appear on the lowest leaves and may first appear as purple spots on stems that eventually turn black. The area around the spots turns yellow and the spot may coalesce to form black blotches (Figure 3 A, B, C). The yellow leaves easily fall off the plants. The disease spreads from lower leaves to younger upper leaves leading to further defoliation. Severe defoliation reduces vigor of the plants and decrease flower production (Gachomo et al., 2010).

Life cycle of blackspot causal agents
Blackspot disease is caused by a fungal pathogen, Diplocarpon rosae. The fungus overwinters on infected canes and fallen debris (Gachomo, 2005). During the favorable weather conditions the spores are splashed from infected plant parts to young leaves by rain splash and irrigation water. The fungus produces conidia within 10 to 14 days (Figure 4 A, B) which are splashed to other young leaves. Several disease cycles can occur within a growing season. Once established the disease is difficult to control.

Control of blackspot disease
Control practices start with planting resistant rose varieties where available. Good sanitation is key to keeping the rose disease free. Recommended cultural practices are: All infected debris should be collected and burnt or buried; all infected canes must be pruned; overhead irrigation must be avoided because it tends to splash conidia from infected to non-infected parts of the plants. It recommended to water plants at the base; the plants should be preferably watered in the morning as opposed to the evening, because the conidia require several hours of wetness to cause infection, therefore watering in the morning reduces the hours of leaf wetness. In addition, plants should be well spaced and kept weed free to allow for aeration. Furthermore, one must avoid planting susceptible plants under the shade. When blackspot disease is establish, its control relies heavily on fungicides. In Table 1

Bottom rot disease of lettuce
Bottom rot disease of lettuce can be recognized by brown spots that initially appear on the midribs of the lower leaves that are in contact with the soil (Figure 5 A). The rot spreads rapidly under favorable conditions to affect larger sections of the midrib and leaf blades, and may affect the inner leaves of the head. Symptoms are more severe during heading.

Fungal agent of bottom rot disease
Bottom rot is caused by a soilborne fungal pathogen Rhizoctonia solani. The fungus overwinters in the soil or in crop debris as sclerotia or mycelia. It may survive in alternate hosts and serve as a source of inoculum, sexual spores. It is disseminated by wind or rain splash in the next growing season. R. Solani has a wide host range e.g. eggplant, soybean, potato, cotton, alfafa, maize, wheat and several weed species.

Control of bottom rot disease
Cultural measures includes three year rotations with non-host plants; collecting plant debris and burying it or plowing it deep in the soil; planting varieties that have an upright architecture to reduce contact with the soil; keeping the fields weed free and removing volunteer crops to reduce possible alternate hosts. Since R. Solani is able to survive on non decomposed organic matter, it is important to avoid planting lettuce in a field that has high amounts of organic matter that is not decomposed; avoid overhead irrigation during heading of the plants; plant lettuce on ridges which increases aeration and helps avoid plants contact with the soil. Fungicides (

Fusarium wilt of lettuce
Lettuce seedlings affected by this disease wilt and ultimately die, while in mature plants the symptoms include red-brown to black discoloration of internal taproot and crown tissue, yellowing of leaves, tipburn of heads (Figure 5 B) and when infection is severe plants are stunted and may fail to form heads.

Fungal pathogen of Fusarium wilt disease of lettuce
Fusarium wilt of lettuce is caused by a soil-borne fungus, Fusarium oxysporum f.sp. lactucae forma specialis nov. This pathogen can remain viable in the soil for many years. Fusarium oxysporum f.sp. lactucae forma specialis nov is host specific to lettuce and therefore only affect/grow on lettuce.

Fusarium wilt disease control
Recommended cultural practices include clean of farm equipment, avoiding to plant lettuce in infected field and planting resistant/tolerant lettuce varieties.

Late blight disease
Late blight is one of the most destructive diseases of potato and tomato. It is found wherever these crops are grown. On potatoes it appears as small light green water soaked spots at the edges of leaves. During favorable weather conditions, cool and moist, the lesions enlarge rapidly, and turn brown to black (Figure 6 A, B). The lesions coalesce to cover entire leaves and even affect the stem. Infected tissue dries up when the weather is dry. The disease spreads rapidly and all the leaves may be killed in a few days. On tubers, the disease appears as irregular, dry, brown depressions. Copper brown, granular lesions are found underneath the skin (Figure 6 A). Potatoes infected with the late blight pathogen are generally susceptible to secondary infection from other fungi and/or bacteria.

Fungal pathogen of late blight disease
Late blight disease is caused by a fungal pathogen, Phytophthora infestans. The primary sources of inoculum are infected seed tubers, volunteer plants and plant debris. Spores are www.intechopen.com dispersed by wind and water splash from infected to non-infected plants. The disease spreads rapidly at temperatures between 10 and 21°C in combination with high humidity. Several strains of the fungus have been reported and strains recombination increases the chance of having novel strains that are either resistant to fungicides or more tolerant to harsh environmental conditions. P. infestans also infects tomatoes and causes mild infections on eggplants, peppers and related weed species.

Late blight disease management
It is recommended to destroy all volunteer potato and other susceptible plants because P. infestans survives on these volunteer plants that represent the primary sources of inoculum during the next season. Potato growers should only use certified seed potatoes and avoid using their own grown tubers as seed in order to contain the devastating effect of late blight disease. It is advisable to make sure that other crops that can also be infected by P. infestans are disease free. Cull piles of infected potatoes should be destroyed because they serve as a source of inoculum. The fields should be scouted for late blight on a regular basis, paying close attention to low lying areas, areas under shade, or near water sources. It is important to avoid overhead irrigation in the evening because this provides long periods of leaf wetness that favors disease development. Potato tubers should be harvested after the vines die, which also kills the spores on them and avoids transmission of spores to the tubers. Infected tubers should be removed before storage in order to avoid spreading the disease to the healthy tubers. Planting resistant or moderately resistant potato varieties where available is advisable.

Chemical control of late blight disease
The fungicides recommended for use against late blight disease vary from region to region because strains of P. infestans found in one region might not be present in another, and fungicide sensitivity might be different among fungal isolates. Genotypes of P. infestans have been reported to recombine to produce new genotypes that are resistant to the recommended systemic fungicides, but resistance to protectant fungicides has not been reported.
In fields that have already been reported to have late blight, the first application of a protectant fungicide is recommended before row closure and a second application should follow within 7-10 days. Further applications of protectants should be done when the weather conditions are conducive for late blight development. A late blight epidemic is difficult to control, therefore regular applications of protectants during the growing season is important to keep new foliage covered. Applications should be made even late in the season as long as parts of the vines are still green to avoid tuber infections. For a complete list of fungicides recommend in a region, it is advisable to consult the area extension office. However, we highlight in Table 3 some of, the recommended fungicides used to control late blight disease on potatoes.

Early blight disease of potato and tomato
On potato and tomato foliage early blight appears as brown to black spots, which coalesce to form lesions that are restricted by large veins and therefore having an angular shape ( Figure  6 C, D). Occasionally, a chlorotic border may be formed around the lesions. When stems are infected the disease appears as small dark spots. On tubers there are dark sunken lesions that are surrounded by raised margins. The tissue underneath the lesions is dry, reddish brown in color, and leathery in texture.   www.intechopen.com

Fungal pathogen of early blight disease
Early blight disease is caused by a fungus, Alternaria solani. The fungus overwinters in plant debris, infected tubers, soil and on other host species. Disease development is favored by temperatures between 20°C and 30°C; long periods of leaf wetness, high relative humidity under alternating wet and dry conditions. Spores are dispersed by wind, water splash, insects, machinery and animals. The disease occurs late in the season and increases rapidly during flowering and senescence. Both biotic and abiotic stresses favor disease development. Bruising or wounding of tubers during harvest leads to infection with early blight.
On tomato the disease symptom is characterized by lesions with dark concentric rings. Diseased leaves wither, dry and fall off. Severe defoliation reduces plant vigor and exposes tomato fruits to sunscald. Disease is first observed on the lower leaves and spreads to the upper leaves. Other symptoms include damping-off, collar rot, stem cankers, leaf blight, and fruit rot.

Early blight disease management
The following cultural practices that promote a healthy crop and therefore hinder early blight disease establishment include: three year crop rotations with non-susceptible crops; removing volunteer crops and keeping the field weed free; planting resistant/tolerant varieties; removing plant debris or burying it in the soil; irrigating in the morning so that the plant have enough time to dry; keeping the plants healthy so that they are less susceptible to disease; having proper spacing between the plants and rows to provide for good air circulation; using certified disease-free tomato seed and transplants; planting potatoes away from previous season potato fields; avoiding bruising and wounding of tubers during harvesting.

Fungicides use in management of early blight
On potatoes it is recommended to apply protectant fungicides at beginning of flowering or at the earliest symptoms of early blight. On tomatoes fungicide application is recommended soon after transplanting or two to three weeks after emergence. In Table 4, the recommended fungicides used in early blight disease control are summarized.

Black scurf disease of potato
On underground stems and stolons the disease appears as brown to black sunken lesions that cause the plants to look weak. These lesions may girdle the stolons and cut them off from the rest of the plant. Lesions that girdle the main stem cause the leaves to turn purplish or yellowish and curl upwards. Other symptoms include formation of aerial tubers and formation of whitish mold on the stems at the soil line. On tubers the disease causes tubers to crack or get deformed. Overwintering structures formed on surface of tubers appears as dark masses or as netted residues.

Fungal causal agent of black scurf disease
Black scurf of potatoes is cause by a fungal pathogen, Rhizoctonia solani Kuhn. The fungus overwinters in the soil on plant debris or inform of sclerotia. Sclerotia may also survive on  Table 4. Fungicides for early blight control in tomato

Disease management
Recommended cultural practices in management of black scurf of potatoes include planting certified disease free seed, planting in warm soils (16°C); warming the seed before planting; rotation with non-host plants such as grasses; avoiding field with a history of disease because the fungal population builds in the soil when potatoes are grown in the same field.

Acknowledgement
We gratefully acknowledge data availability by colleagues from Research units and Extension services from the University of Maine, Idaho, Pennsylvania State University, www.intechopen.com