Bacterial leaf spot (BLS) has been identified as the most important problem of peppers in the eastern United States. Leaf spots are water-soaked initially, then turn brown and irregularly shaped. Affected leaves tend to turn yellow and drop. Yield is reduced because loss of foliage reduces plant productivity and leaves fruit vulnerable to sunscald, and also because raised, scab-like spots may develop on fruit. Complete crop failure has occurred.
Information on the biology of a disease is essential for designing a management program. The first strategy to use for controlling any disease is to eliminate or reduce the amount of the pathogen available to initiate disease. This requires knowledge of how the pathogen survives in the absence of living host crop plants. The most important source of the bacterium that causes leaf spot in pepper is infested seed. Infested crop debris and infected weeds are additional sources. Tomato is also susceptible to leaf spot. The strain of the bacterium that infects tomato causes small spots on pepper.
One of the most important management practices for leaf spot, therefore, is to use disease-free seed or transplants. Only hot-water treatment can kill bacteria inside seed; chemical controls are effective only for pathogens on the seed surface. It is best to have the seeds custom treated because it is possible that hot-water treatment can adversely affect germination if proper precautions are not taken. Furthermore, if you treat the seed, the seed company's liability and guarantees are null and void. If you decide to do it yourself, treat at 122o F for 25 minutes. A precision laboratory thermometer is needed to ensure the temperature is correct. It can be purchased from a laboratory supply company such as Fischer Scientific (800-766-7000) for about $10 (catalogue no. 14-983-15B). While treatment can be done in a pot on a stove, it is easier to maintain temperature with a stirring hotplate plus stir bar., which cost about $300 (no. 11-498-7SH) and $5 (no. 14-511-65), respectively. Have a container of cool water handy to prevent the temperature from getting too high. Make sure the hotplate is maintaining the water at 122oF before beginning. Wrap the seed loosely in cheesecloth or a piece of cotton cloth and add a metal bolt or sinker to keep the seed submerged. Check the temperature constantly. Upon removing, cool the seed under tap water. Spread the seed out on paper towels to air dry at 70-75oF. It is essential to do a small quantity of seed and check the germination against nontreated seed of the same lot number, before treating all seed.
In addition, use at least a 2-year rotation because the pathogen can survive in infested crop debris. 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.
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 (copper fungicides are toxic to bacteria), or avoiding conditions that enable the pathogen to spread and multiply rapidly.
Information about the biology of the pathogen is useful when selecting resistant varieties. Five races of the pathogen have been identified; four are in the United States. Race 1 occurs throughout the United States. Race 2 is found in Florida and the Caribbean. Race 3 is a common mutant of both races 1 and 2. Races 4 and 5 are uncommon and expected to remain so because they are not fit. Races 1 and 3 were detected in CT and MA in 1997. Several commercial varieties have resistance to races 1, 2 and 3, including Boynton Bell (Harris Moran), Commandant (Rogers NK), Enterprise (Asgrow), Goldcoast (Asgrow), Summer Sweet 870 and 880 (Abbott & Cobb), and X3R Aladdin, X3R Camelot, X3R Lancelot, and X3R Wizard (all from Petoseed). X3R Aladdin mature yellow: however, X3R Aladdin matures too late in the northeast to grow to maturity. Varieties with resistance to only races 1 and 2 were as severely infected as varieties with no resistance in an experiment with races 1, 2 and 3. There can be substantial differences in yield between resistant and susceptible varieties grown in the presence of BLS: fruit weight from Boynton Bell was 2.6 times greater than Camelot in one experiment and 29 times greater than North Star in another experiment. Research has also shown that resistant varieties can be more effective than bactericides for controlling BLS and preventing yield reduction.
A chemical control program is recommended for susceptible varieties, and should begin in the early stages of disease development. Streptomycin (Agri-Strep) can be applied before transplanting beginning at the first true leaf stage and continuing every 3 to 5 days. Some feel this is worthwhile: others question the need if seed is hot-water treated. Development of resistance is a concern with streptomycin. Examine the plants every week before and after transplanting. It is worthwhile to remove infected plants if they are found early before there has been the opportunity for spread. After transplanting, apply bactericides every 7 to 10 days; use the shorter interval when rain and warm temperatures occur. Research has shown that BLS can be managed with an IPM program (bactericide applications started after disease detection through weekly scouting and delayed when less than 60o F at night) as effectively as with a preventive spray program. Copper plus maneb is reportedly more effective than copper alone because more copper goes into solution. Let it sit for about 90 minutes with agitation before spraying. However, combining Maneb with Kocide 2000 did not improve efficacy in a recent experiment. Harvesting is restricted by a 7-day PHI when Maneb is used. An airblast sprayer should not be used because it can disperse the bacteria. Discontinue spraying in late summer when night temperatures are consistently below 60oF 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. Overhead irrigation provides favorable conditions for disease development and spread of the pathogen whereas trickle irrigation does not promote disease development. Disk the field to begin breakdown of crop residue as soon as possible.
In summary, the two most important management practices for BLS in pepper are hot-water seed treatment and resistant varieties. In addition, use at least a 2-year rotation, rogue infected plants, use trickle irrigation, scout weekly, and disk the field immediately after harvest. If necessary, apply copper plus maneb after disease detection on a 7 to 10 day schedule.
The specific directions on fungicide labels must be adhered to -- they supersede these recommendations, if there is a conflict.
See other articles on the UConn
Vegetable IPM site:
"Preventing Bacterial Diseases of Vegetables
With Hot Water Seed Treatment"
"Bacterial Leaf Spot"
Prepared by: Margaret
Tuttle McGrath,
Associate Professor, Department of Plant Pathology,
Long Island Horticultural Research Laboratory, Cornell University.
Reprinted from Proceedings. 1997. New England Vegetable & Berry Conference and Trade Show. p. 64-65
This information was developed for conditions in the Northeast. Use in other geographical areas may be inappropriate.
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