Managing Bacterial Leaf Spot

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 and drop (Figure 2). Yield is reduced because raised, scab-like spots may develop on fruit (Figure 3) and because affected leaves drop off plants (figure 4), 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. The most important source of the bacterium that causes BLS in pepper is infested seed. Therefore, the use of disease-free seed or 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®; 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, if you treat the seed, the seed company’s liability and guarantees are voided. If you decide to do it yourself, treat at 122oF for 25 minutes. The best way to control temperature while treating seed is to use a stirring hot plate and a precision laboratory thermometer. Hot plates and thermometers can be purchased from a laboratory supply company such as Fischer Scientific (800-766-7000). Hot-water treatment can be done successfully using a large pot on a stove top and a precision laboratory thermometer.

With either equipment, expect to spend some time adjusting settings to achieve a constant 122o F, especially with the stove top. A very low hot plate or stove setting will ultimately 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 122o F. 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 for stirring when using a stove top. 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. Upon removing, cool the seed under tap water. Spread the seed out on paper towels to air dry at 70-75o F. It is essential 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® Regular Bleach now has a label for pepper seed treatment (EPA Reg. No 5813-1). 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. Some seed companies routinely use chemical seed treatment, which may be described as ‘calcium hypochlorite’ on the label. To Clorox®-treat seed: mix 1 quart Clorox® with 4 quarts water, put up to 1 pound of seed in a cheesecloth bag, submerge in solution and agitate for 40 minutes, rinse seed under running water for 5 minutes, then dry it thoroughly on a paper towel in a location free of mice. 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, a label with this use must first be obtained from the Clorox® company (800-446-4686).

Either seed treatment should be done within a few weeks of planting. Afterwards dust seed with an effective fungicide (i.e. 1 teaspoon Thiram 75W per pound of seed) to prevent damping-off and other pathogens from infecting seeds.

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. Keep the greenhouse as dry as possible and minimize 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 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.

Choose varieties with resistance to as many BLS races as possible. Resistance is currently available for races 1-3. New races (4-6) have been discovered in southern states. Recent BLS survey results in the Northeast have failed to detect the new races in our area. Thus, resistant plants should still be considered one of the best management strategies available to combat BLS. 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 preventative 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 for many years with good results.

Adding maneb (i.e. Manex) to a copper application has been shown to 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 61°F 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. Reduce favorable conditions in the greenhouse by keeping it as dry as possible and by minimizing splashing water. 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 was 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.

By: Margaret Tuttle McGrath, Associate Professor, 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. McGrath photos.

Updated by: T. Jude Boucher, IPM, University of Connecticut. 2012

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