Bacterial Leaf Spot in Peppers

The University of Connecticut Cooperative Extension System started to develop and deliver a full-season IPM training program to commercial pepper growers in 1989. The program helps farmers recognize and successfully manage most of the important pests that influence the economics of producing this crop. By far, the most destructive pest of peppers in Connecticut, New England, and the throughout the East Coast during the past seven years has been bacterial leaf spot (BLS). Since 1989, BLS has been observed on 90% of Connecticut’s pepper plantings. Up to 91% of the acreage enrolled in the IPM program has been infected with this disease in any given year. Ironically, only 24% of the IPM growers identified BLS as a significant pepper problem in pre-program surveys. Before pepper production can be profitable, New England growers must learn to recognize this disease and minimize its occurrence and destruction.

Over 2/3 of the pesticide applied to peppers by Connecticut growers was targeted at controlling this disease, yet nine out of 10 producers still lost a substantial portion of their yield and profits to BLS. Although most IPM participants have yield increases after program involvement, there is a lot of room for improvement in managing this disease. Why is BLS so difficult to control? Bacterial diseases on most crops are notoriously hard to control, especially in places with humid, wet climates that favor disease infection and spread. Bacteria multiply rapidly, and can be spread throughout the field within a week’s time by any mechanical means you can imagine: on worker’s hands, by splashing rain drops or irrigation water or by machinery such as cultivators. Many of the strains of the Xanthomonas campestris pv. vesicatoria bacteria that cause BLS are completely or partially resistant to copper and antibiotics; the only two pesticides capable of partially “controlling” the disease. If weather conditions favor disease development and spread, pesticides are ineffective at stopping even susceptible strains of the bacteria. Low foliar nitrogen and potassium, and high magnesium and calcium levels have also been associated with increased crop susceptibility to BLS.

Some studies have shown that high-pressure mist blowers or airblast sprayers actually increased BLS incidence on plants in rows close to the nozzles by forcing bacteria present on the surface of foliage into the leaf. Such high pressure is also thought to increase the number of entry sites for infection because of broken leaf hairs and abrasion from windblown particles. High-pressure sprayers may also help spread the bacteria across rows on contaminated water droplets and crop residue.

Disease Symptoms and Development

Bell types are particularly vulnerable to this disease, although under favorable conditions for BLS spread, more tolerant varieties of hot or sweet peppers may show varying degrees of susceptibility. The X. campestris bacteria can be introduced on infected seed or arrive on transplants. It cannot survive in the soil for extended periods, but will live through the winter on the residue of solanaceous crops (especially peppers and tomatoes), on weeds in the nightshade family and on the roots of wheat. There are three known races of the leaf spot bacteria and they differ in their ability to infect various breeding lines of peppers (varieties) and the symptoms that they produce.

The bacteria may enter leaves through natural openings (i.e. stomates) or through abrasions or broken leaf hairs. The presence of water on the leaf surface is important in the infection process and thus, the first symptoms usually occur on the margins or tips of leaves where droplets accumulate. Once inside the plant, the bacteria reproduce rapidly and move through the vascular system, congesting and killing tissue. Symptoms may develop in as little as five days or not show up for six or more weeks.

BLS affects all aboveground parts of the plant. Spots on leaves appear as chocolate-brown irregular-shaped areas of dead leaf tissue. These spots start out less than 1/4 inch in diameter, grow in size and accumulate in number until the infected leaves drop off. In a severe outbreak, plants will drop 50 to 100% of their foliage, leaving the fruit exposed to the sun and vulnerable to sunscald. Plant stems and branches develop elongated, raised, light-brown cankers, less than 1/4 inch long. Fruit may develop light-colored, raised, wart-like spots that eventually darken to brown or black. BLS spreads rapidly up the crop row after canopy closure and can usually be distinguished from fertilizer burn or other problems because several adjacent plants will share similar symptoms.

Disease development is favored by relative humidity above 85%, extended periods of leaf wetness and heat waves, especially when night temperatures remain above 70 degrees F. Short periods (three days or more) below 40% relative humidity will reduce disease severity and delay development. Extended periods (three weeks) of low humidity irreversibly halt disease spread and development, even if favorable conditions return later. In addition, night temperatures below 61 degrees F suppress disease development. Typically in New England, both our humidity levels and our night temperatures begin to drop dramatically by mid-August. Depending upon the late summer weather in a given year, chemical controls may be safely dropped without fear of further disease spread. In 1995, all pepper growers on the Connecticut IPM program stopped spraying for BLS in mid-August, and experienced no increase in disease severity.

Integrated Disease Management

Once BLS arrives on your farm, it will claim at least some of your yields and profits, even if chemical control is “successful.” Too many times, I have seen it destroy 80-100% of a grower’s peppers, taking several years-worth of profits at once. If inoculum (bacteria) levels are high enough, with the right environmental conditions, even “resistant” varieties may be hurt or succumb to this disease. Good sanitation and preventative practices are essential to minimize the risk of initial infection and reduce the amount of bacteria present early in the disease cycle. Delaying the spread of this disease for a week or more early in its development, may add weeks to your harvesting time and translate directly into higher profits.

Start by washing and disinfecting used bench tops, flats and cell trays and sterilizing soil for seedling production or using synthetic media. Do not accept transplants from other farms unless proper sanitary practices are followed. Relocate or fumigate seedbeds annually. Choose fields that have been free of solanaceous crops or weeds and wheat for at least two years. Choose sites that hasten the drying of foliage after rain or irrigation and avoid excessively foggy areas. Use trickle irrigation instead of overhead. Scout fields at least weekly to detect the presence of BLS as soon as possible. If detected in the early stages, when just a few localized plants show symptoms, remove or bury all diseased specimens and symptomless plants for 10 or more feet in all directions. Always work in infected sections of pepper fields last and disinfect machinery after completing the job. Deep plow crop residues after final harvest to speed decomposition of infected tissues.

Soils amended with dolomitic lime (high in magnesium) have been shown to produce a higher incidence of BLS on pepper plants than when the soil was adjusted with Cal limestone (CaCo3). If possible, switch to calcium carbonate (Cal limestone) several years before planting peppers in a field. Maintain proper nutrient levels, especially nitrogen, by sidedressing with ammonium nitrate or urea several times during the season.

Seed Treatments

Companies take many preventative measures to reduce the spread of BLS, including; inspecting production fields, testing 10-30,000 seeds per lot for bacterium or using chemical treatments, such as sodium hypochlorite, to disinfect the surface of the seed. Some companies offer hot-water seed treatment for a small fee. Research has shown that hot-water treatment can penetrate the seed sufficiently to eradicate bacteria inside the seed. Since every plant in a field can eventually become infected with BLS from a single infected seed, hot-water treatment is highly recommended. It is easier, cheaper and more effective than trying to combat BLS in the field with chemicals, but there is a risk of reducing germination. Seed crops grown under stressful conditions may not tolerate the treatment as well as seed from plants that were not stressed.

Once you hot water treat, all seed company liability and guarantees are null and void, therefore, the following precautions and recommendations should be observed. It is important that the water be maintained at a uniform temperature throughout the vessel; use a stirring hot plate not a stove. Use a laboratory thermometer to assure that the water temperature does not rise above 122 degrees F, and treat for 25 minutes only, no longer. Watch the temperature throughout the duration of the procedure.

Conduct a greenhouse germination test before exposing all your pepper seeds to the high temperature bath. Treat a 100 seed sample of each variety and lot number and plant alongside of an equal number of untreated seed in the same growing media that you plan to use for your transplant production. If the test gives acceptable germination rates, treat as much seed as you expect to plant this year, carefully using the same procedure. Apply a fungicide according to the manufacturer’s directions for protection against soil-borne rot organisms.

We tried this to make sure it is safe when done correctly. In the spring of 1995, germination tests were conducted on five new lots (varieties) of pepper seed and one lot which was a year old. The hot water treated seed had an average germination rate of 95% compared to 94% for the untreated seeds when planted the day after treatment. Ten weeks after the hot-water treatment a second planting was made, and again, there was no reduction in germination for treated seed.

For more details see ‘Preventing Bacterial Diseases of Vegetables with Hot Water Seed Treatment‘ by Jude Boucher, Ruth Hazzard and Robert Wick. Contact Jude Boucher at UConn’s Vernon Cooperative Extension office.

Pesticides and Sprayers

Some states recommend the use of streptomycin on seedlings before transplanting, but this often produces unsatisfactory results due to the prevalence of resistance to antibiotics. Growers on the IPM program apply copper plus Manex every seven to 10 days once the disease is detected through weekly field scouting. The fungicide Manex has no direct effect on the bacteria but has been shown to double the amount of copper that goes into solution when the two materials are mixed. The copper in solution increases fourfold within an hour and sevenfold in four hours, if the mixture is allowed to age before application. Manex is one of the few maneb type products still labeled for use on peppers and contains zinc, which has been shown to suppress copper resistant strains of the bacteria.

Low-pressure electrostatic sprayers or booms with drop nozzles will provide the best spray coverage and protection for many highly contagious diseases on short row crops.

Resistant Varieties

Many new resistant bell pepper varieties are now available. I advocate a complete shift to resistant varieties as soon as possible to minimize losses from BLS and maximize profits over the long haul. See ‘Disease Resistant Pepper Varieties’ in this publication for more details.

Putting It All Together

None of the above mentioned strategies if used alone will provide 100% protection of your crop when environmental conditions favor BLS and high levels of bacteria are present. There are no magic bullets! Only by integrating key components such as hot-water seed treatment, proper sanitation, resistant varieties, weekly scouting and chemical control (if necessary), can you grow peppers profitably each year.

By: T. Jude Boucher, Vegetable Crops IPM Program Coordinator, University of Connecticut Cooperative Extension System, 24 Hyde Ave., Vernon, CT 06066. Reviewed 2012.

Published in: Proceedings. 1995 New England Vegetable and Berry Conference and Trade Show. December 12-14, 1995. Sturbridge Host Hotel, Sturbridge, MA. Pp.144-147

This information was developed for conditions in the Northeast. Use in other geographical areas may be inappropriate.

The information in this document is for educational purposes only.  The recommendations contained are based on the best available knowledge at the time of publication.  Any reference to commercial products, trade or brand names is for information only, and no endorsement or approval is intended. The Cooperative Extension System does not guarantee or warrant the standard of any product referenced or imply approval of the product to the exclusion of others which also may be available.  The University of Connecticut, Cooperative Extension System, College of Agriculture, Health and Natural Resources is an equal opportunity program provider and employer.