Managing Bacterial Leaf Spot in Pepper
by Margaret Tuttle McGrath, Department of Plant Pathology, Long Island Horticultural Research and Extension Center, Cornell
and Jude Boucher, Sustainable Agriculture/IPM Educator - Vegetable Crops, University of Connecticut Cooperative Extension System, Vernon, CT
Bacterial leaf spot (BLS) is the most common, and one of the two most destructive diseases of peppers in the eastern United States. Leaf spots are water-soaked initially, then turn brown and irregularly shaped (Figure 1). Affected leaves tend to turn yellow (Figure 2) and drop (Figure 3). Yield is reduced because raised, scab-like spots may develop on fruit (Figure 4) and because affected leaves drop off plants, thereby reducing plant productivity and exposing fruit to potential sunscald (Figure 5). Complete crop failure has occurred. Tomato is also susceptible to BLS. The strain of the bacterium that infects tomato causes small spots on pepper.
The first strategy to use for controlling any disease is to eliminate or reduce the amount of the pathogen available to initiate disease. An important source of the bacterium that causes BLS in pepper is infested seed. Therefore, the use of disease-free seed and transplants are some of the most important BLS management practices. Infested crop debris and infected weeds are additional sources of the pathogen and must also be managed in an effective BLS control program.
Seed can be treated with hot water or Clorox® bleach (calcium hypochlorite) to kill the pathogen. Hot-water treatment is more thorough than Clorox because it reaches inside seed; however, high temperatures can adversely affect germination if proper precautions are not taken. It is best to have seed custom treated, which some seed companies will do. Furthermore, it you treat the seed, the seed company’s liability and guarantees are null and void. If you decide to do it yourself, treat at 122o122oF for 25 minutes. The best way to control temperature while treating seed is to use a precision laboratory water bath. Procedures are described in an article on hot water seed treatment. If great care is taken, hot-water treatment can be done successfully using a large pot on a stovetop and a precision laboratory thermometer (Figure 6).
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With either equipment, expect to spend some time adjusting settings to achieve a constant 122oF, especially with the stovetop. A very low hot plate or stove setting will probably provide the correct temperature. Wait to begin treatment until the water temperature in the pot is maintained. Have containers of hot and cold water nearby in case the water temperature does not stay at 122oF. Place seed in a tea infusion ball or in a piece of cotton cloth. Add a metal weight to keep the seed container submerged, but make sure it is not on the pot bottom. Agitate the water continuously. A wooden spoon works well when using a stovetop. Check the temperature constantly. Keep the thermometer off the hot bottom of the pot; this can be accomplished by taping it to the inside of the wooden spoon used for stirring (Figure 6). Upon removing, cool the seed under tap water. Spread the seed out on paper towels to air dry at 70-75oF. It is recommended to conduct a preliminary germination test with a small quantity of treated and untreated seed from each variety and lot number before treating all the seed. Some seed lots produced from stressed plants may not stand up to hot water treatment and germination may be adversely affected (though this is rare with pepper seeds).
Clorox Commercial Solutions® Clorox® Germicidal Bleach (EPA Reg. No 5813-100) and Clorox Commercial Solutions® Ultra Clorox® Germicidal Bleach (EPA Reg. No 67619-8) are labeled for pepper seed treatment. There is less chance of seed being damaged with bleach than hot water; however, chemical controls such as Clorox are effective for pathogens on the seed surface only; hot-water treatment can kill bacteria inside as well as on the outside of seed. These have 7.85% and 5.84% available chlorine, respectively. To Clorox treat seed, prepare a solution with 10,000 ppm available chlorine. Mix 16.7 or 22.2 fl oz of these products, respectively, with 1 gallon of water to obtain treatment solution. Use 1 gallon of this solution per pound of seed. Put up to 1 pound of seed in a cheesecloth bag, submerge in this solution and provide continuous agitation for 40 minutes, rinse seed under running tap water for 5 minutes, then dry seed thoroughly on paper towel. Put the seed in a new package, not back in the original one. Prepare a fresh batch of the dilute Clorox solution for each 1-pound batch of seed. The soak can stimulate germination, so if the seed is dried and held too long, germination will be reduced. To legally make this treatment, only these formulations can be used and the full label with this use must first be obtained from the Clorox company (800-446-4686) or by going to the following web site.
Enter ‘Clorox Germicidal Bleach’, for ’Name’ in second section on the right side. Click on ‘Search’ below the search section. When the results are displayed, click on the ‘More’ button in the lower right corner of the box for either product. Note that the search results will include other products that do not have use directions for seed treatment.
Either seed treatment should be done within a few weeks of planting. Best is doing right before planting as treatment can prime seed for germination. Afterwards a fungicide can be applied to prevent damping-off and other pathogens from infecting seeds. There are several formulations of Thiram registered for application as a dust or slurry.
Other management techniques which help eliminate or reduce the initial amount of disease pathogen present include greenhouse sanitation, producing your own disease-free transplants, crop rotation, proper weed control and hastening the decomposition of host plant residue. Grow your own transplants under sanitary conditions to avoid importing bacterial leaf spot on seedlings purchased from off-farm sources. Always start with new or disinfected greenhouse supplies and materials when planting peppers. Examine seedlings weekly for symptoms. Reduce favorable conditions for BLS in the greenhouse by keeping it as dry as possible and by minimizing splashing water. In the field, use at least a 3-year rotation because the pathogen can survive in infested crop debris until it completely decomposes. Do not rotate pepper with tomato, eggplant, or potato and do not grow these crops together. Control nightshade, horsenettle, jimsonweed, and all other solanaceous weeds in current and future pepper fields. Disk or plow the field immediately after the final harvest to hasten the breakdown of crop residue.
The second disease control strategy is to reduce the rate at which the disease develops in a planting. This can be accomplished by selecting resistant varieties, applying bactericides if necessary, and/or avoiding conditions that enable the pathogen to spread and multiply rapidly. An integrated management program for BLS is recommended to ensure effective control as bactericides or resistant varieties alone will not be sufficient when conditions are very favorable for BLS (hot and wet) or when less susceptible BLS races are present.
BLS resistant pepper varieties usually provide effective control of BLS and excellent yield. Resistant varieties have performed well in several experiments. For example, susceptible Merlin and North Star were severely infected by BLS and produced only 0.7-1.4 ton/A of marketable fruit while resistant Boynton Bell produced 20.5 ton/A (KY, 1995). All 7 resistant varieties tested on Long Island provided better control of BLS than a weekly preventive spray program of the copper fungicide/bactericide Kocide 2000 + Maneb 75DF (which was applied with a tractor-mounted boom sprayer delivering 100 gpa at 250 psi). These varieties yielded as well as the susceptible standard Camelot treated preventively. Applying Kocide + Maneb to the resistant varieties did not improve disease control or increase yield in these experiments. However, in Florida, where conditions can be very favorable for BLS, growers feel they need to use foliar sprays on resistant peppers.
Effective control of BLS with resistant varieties requires knowing what races of the pathogen are likely to be present. BLS resistant varieties have race-specific resistance. Thus a variety lacking resistance to all of the races present may not perform any better than a susceptible variety. Race 1 occurs throughout the United States. Race 2 is found in Florida and the Caribbean. Race 3 is a common mutant of both race 1 and 2. Races 4 and 5 are uncommon and expected to remain so because they are not fit. Races 1 and 3 have been found in the northern U.S.; therefore varieties selected for this area need resistance to both races 1 and 3. The situation could change in the future. A widespread and serious outbreak of BLS caused by races 4 and 6 occurred throughout southern Florida, where most varieties grown have resistance to races 1 - 3, during the unusually wet 19971998 winter vegetable season. Unfortunately, resistance is only available for races 1 - 3 at present. Resistant varieties should be examined routinely for symptoms in order to detect occurrence of new races early enough in disease development to initiate a chemical control program. Results from variety trials also should be considered when selecting resistant varieties as they differ in fruit quality and yield.
A chemical control program is recommended for susceptible varieties that show symptoms, and should begin in the early stages of disease development. Examine the plants every week. It is worthwhile to remove infected plants if they are found early, before there has been the opportunity for spread. If infected plants are found while scouting, apply bactericides (copper fungicides are toxic to bacteria) on a 7 to 10-day schedule; use the shorter interval when rain, high humidity and warm temperatures occur. Stretch the interval by one day for each night that temperatures fall below 61oF. Recent research on Long Island showed that a program where applications began after disease symptoms occurred, was less expensive and worked as well as weekly preventive sprays initiated early in the season before the disease was present. Connecticut growers have used weekly scouting and this "wait-and-see" policy before implementing a spray program with good results for many years. Adding maneb to a copper application has been shown be more effective than using copper alone in some experiments. Efficacy is improved because more copper is dissolved when copper and maneb are agitated together in the spray tank for about 90 minutes before application. However, maneb may not improve efficacy of newer copper formulations that provide more available copper in the spray solution than older products. A high pressure airblast sprayer should not be used because it can disperse the bacteria through a field. Discontinue spraying in late summer when night temperatures are consistently below 61oF and daytime relative humidity is well below 85%. While chemical control can be effective, it is not foolproof. Failure can occur when conditions are very favorable for disease development and when plants are infected with a bacterial strain that is resistant to copper.
Warm, wet conditions are favorable for diseases caused by bacteria. Therefore, irrigation method is an important consideration in managing BLS in the field. Overhead irrigation provides both a means of spread for the pathogen and favorable conditions for disease development, therefore trickle irrigation is recommended. In addition to movement by splashing water drops, the pathogen can be spread through any mechanical means imaginable when plants are wet, including on workers hands and on machinery such as cultivators. If possible, avoid working fields when the plants are wet and work infested areas last. Disinfect machinery used in infected sections of the field after the job is completed.
Low nitrogen or potassium, and extra high magnesium and calcium levels have been associated with increased crop susceptibility to BLS. Pepper crops that show visible signs of nitrogen deficiency (light colored leaves) have been severely affected by BLS in Connecticut. Researchers have also found that BLS is more severe on pepper plants grown in soils adjusted with dolomitic lime, which is high in magnesium, than plants grown in soils adjusted with Cal limestone (CaCO3). Maintain nutrients at the proper levels (moderate to high) to help plants resist infection.
In summary, some of the most important management practices for BLS in pepper are using hot-water seed treatment, planting disease-free transplants, selecting resistant varieties, and using a 3-year rotation to prevent re-infection. In addition, use trickle irrigation, scout weekly, rogue the first infected plants found, maintain proper nutrient levels, use proper sanitary measures and disk or plow the field immediately after harvest. If necessary, apply copper (or copper plus maneb) after disease detection on a 7 to 10 day schedule with a boom sprayer.
The specific directions on fungicide labels must be adhered to -- they supersede these recommendations, if there is a conflict. Any reference to commercial products, trade or brand names is for information only, and no endorsement or approval is intended.