Thermotherapy to free seeds from pathogenic bacteria.
In present day agriculture, use of chemicals for crop production is discouraged. Hence, other alternative treatments for disease control must be developed, and hot water treatment is one of them. It is a feasible practice, both financially and time wise. Hot water soaking is a very age-old practice, efficient in destroying pathogens borne both outside the testa and inside the seed testa by using temperature hot enough to kill the organism but not quite hot enough to kill the seed. Extensive research work has been reported on hot water treatment in vegetables. Therefore, an attempt has been made to review the information available regarding the effect of hot water treatment on growth, disease incidence and yield of vegetables.
- hot water treatment
- seed-borne disease
In the present age when the crop production is protected by chemicals and threat of disease due to exposure to these chemicals is at a rise, organic farming is getting momentum. This organic farming is regulated by certain certifications including chemical-free state of agriculture farm. In this condition use of chemicals will not help farmers for these certifications; rather, there is rejection of the produce. Here it becomes pertinent to involve those practices which are natural and can help control diseases naturally. Priming and pelleting are commonly used practices for seed treatment to enhance the production of
High humidity in the environment and moist soil together with optimum temperature result in high incidence of various diseases. Diseases like bacterial spot (
2. Disease of sweet pepper
Bacterial spot (
3. Seed technology
Seed is the primary and essential starting point of a wide range of horticultural crops, including the majority of vegetables. The high-quality seeds of vegetable varieties that display early, consistent, dynamic seedlings and better- and good-quality fruits from individual seed sown at favourable or unfavourable conditions have increased significantly in recent years. Seedling emergence and field stand establishment is one of the problems faced by the growers, especially in early planting where adverse conditions are prevailing (low temperature and high soil moisture). Delayed, erratic germination and emergence, poor stand, slow early seedling growth rate and non-uniform maturity often limit crop production even under optimum environmental conditions [8, 9, 10]. Extensive seed germination and seedling appearance has increased the occurrence of pre-damping off mortality caused by soil-borne fungi . This also leads to establishment of weeds in the fields even before the crop seedlings are mature enough to be cultivated, competing with the main crop for nutrients, and moreover, they hinder the processes of fertilisation, chemical application and mechanical harvesting.
4. Hot water treatment
The most appropriate seed treatment with respect to least damage, economy, efficiency and application is hot water soaking. It is an old-age practice based on treatment with hot water whose temperature is high enough to kill the pathogen but not high enough to harm the seed, hence a very good technique to control many seed-borne diseases [12, 13]. Heat treatment may be applied for agricultural commodities by (1) immersion in hot water, (2) exposure to vapour heat, (3) exposure to hot dry air, (4) treatment with infrared radiation or (5) microwave radiation. Hot water treatments of seed and plant material are classical thermophysical methods of plant protection and are more eco-friendly and effective than chemical treatments.
Hot water treatment can be damaging or not practical for seeds of peas, beans, cucumbers, lettuce, sweet corn, beets and some other crops [12, 14, 15], but it is highly recommended for pepper, eggplant, tomato, cucumber, carrot, spinach, lettuce, celery, cabbage, turnip, radish and other crucifers. It may also severely damage old seeds, and therefore, a small sample of any seed lot over 1 year should be first treated and then tested for germination to determine the amount of injury that may occur. Hot water treatment is recommended for seeds with surface or deep-seated infections. Effective treatment temperature and duration have to be found out for every vegetable crop and the relevant pathogens. The principle is to eliminate the pathogens as far as possible without decreasing germination of seeds. For example, just a 5-min difference in treatment time can lead to diverse differences in the germination rate of cabbage seed.
A number of tests and studies of heat treatment must be undertaken to optimise the time and temperature that are most adaptable to the seeds to be treated and the pathogens to be killed before practical application. Susceptibility to heat damage may differ among different varieties of plant species [16, 17]. The time/temperature combination for a given plant seed depends on many factors interacting with the heat susceptibility of the host, viz., conditions of external layers, dormancy, moisture content, age and vigour . Particularly, it has long been known that the smaller the initial water content of seeds is at the time of heating, the greater the resistance to high temperatures . Two major groups of proteins may be activated by the hot water treatments that induce fruit resistance: heat shock proteins (HSPs) and pathogenesis-related (PR) proteins. HSPs are believed to play a major role in thermotolerance [20, 21, 22]. Among the PR proteins, most characterised enzymes chitinases and β-1,3-glucanases hydrolyse polymers of fungal cell walls and are, therefore, thought to be involved in the plant defence mechanism against fungal infection [23, 24].
5. Effect of hot water treatment on different vegetables
Nega et al.  stated that even with longer treatment times, hot water treatment with a temperature of 40°C had no significant effect on the seed-borne pathogens. However, on all the crops investigated, hot water treatments at temperatures 50 or 53°C for 10–30 min had a good phytosanitary effect. In the majority of cases, these treatment conditions did not affect seed germination. Therefore, to reduce the effect of higher temperature like 53°C on germination, comparatively shorter treatment time must be used, especially on sensitive crops like cabbage, etc. The treatment of 50°C for 30 min is optimal against
In the past, Walker  observed complete abolition of
Melanie et al.  demonstrated the efficiency of hot water treatment method in reducing bacterial diseases like bacterial spot and bacterial canker in tomato under greenhouse as well as open-field conditions. They also observed that after treating, seedlings from the same seed lots with hot water did not get diseased in the greenhouse or fields. In plots/fields established from hot water-treated seed, the occurrence of bacterial canker was less extensive, and yields were higher than the plots/fields established from non-treated seeds. Also, fruits from non-treated seeds were considerably smaller than fruits from treated seeds. Reduced infection frequency of bacteria responsible for bacterial canker and bacterial leaf spot was observed in tomato seeds after hot water treatment with increased fruit size and yield.
Hot water treatment of seeds of okra (
The effects of hot water treatments of carrot seeds on seed-borne fungi, germination, emergence and yield were studied by Hermansen et al.  where the seeds infected with
Hot water treatment of seeds was also observed to be helpful in controlling seed-borne pathogens in sweet pepper. Aguilar et al.  observed that hot water treatment of bell pepper at 45°C for 15 min or 53°C for 4 min before storing them at 8°C reduced the occurrence of fungal infections. Several hot water treatments of bell pepper seeds resulted in considerable drop-off in seed viability but had no effect on seed vigour . No study can be found in the literature that attempted to arrive at the optimum time-temperature combination for sweet pepper. Therefore, the effect of hot water treatments of sweet pepper seeds on seed viability and seedling vigour needs to be investigated (Table 1).
|Causal agent||Host||Thermotherapy tested||Result||Ref.|
|Tomato (||Hot water treatment at 53, 54 and 55°C for 10–60 min||Germination remained unaffected up to 55°C for 30 min; bacteria were recovered after 30 min at 53 and 54°C and not after 40 min||Bryan |
|Tomato||Soaking infected seeds 30 min in hot water at 56°C||Plants develop significantly less disease than when no seed treatment is used; seed germination is slightly reduced||Shoemaker and Echandi |
|Bean (||Hot water treatments: 50°C for 45–60 min||Reduction of bacterial number by 98–l00% but 45% reduction in seed germination for 60 min soaking with naturally infected seeds||Tamietti ; Tamietti and Garibaldi |
|Pea (||Dry heat at 65°C for 72 h and soaking in water at 55°C for 15 min||Significant reduction of pathogen contamination but germination lowered 5–20%||Grondeau et al. |
|Tomato||Infected seeds are subjected to hot water treatment at 48°C for 60 min||Pathogens are killed, and germination is not affected||Devash et al. |
|Cabbage, cauliflower (||Hot water treatment at 50°C for 30 min||Pathogens are eliminated successfully by this treatment to prevent seedling infestation||Shekhawat et al. |
|Carrot (||Hot water treatment (52°C for 10 min)||Can prevent pathogen infestation||Ark and Gardner |
|Pumpkin (||Hot water treatment at 54 and 56°C for 30 min||Greatly reduces the level of seed infestation but does not completely eliminate it||Moffett and Wood |
High incidences of disease in consumers leading to fatal diseases like cancer have attracted the attention of researchers. The causative agents were explored, and the focus was indiscriminate use of chemicals starting from seed treatment to crop productions. The residual effects of pesticides, herbicides and other chemicals are long lasting, adversely affecting the health of consumers. Some mechanism and treatments are needed to address these problems. Hot water treatment is one such mechanism which has been reviewed in the chapter to help the consumers taking care of all the public health issues. The present study provides easy to practice technique to farmers without involving cumbersome techniques to farmers to protect the capsicum against bacterial, viral and fungal diseases, leading to optimal productions without harming the interest of consumers.