Copper essential in vegetable disease control

Copper is used on nearly 30 vegetable crops to control diseases such as alternaria, anthracnose, powdery mildew, rusts and molds. Moisture is a key culprit in these diseases, although some, such as powdery mildew, can actually thrive under relatively dry conditions. Each pathogen has a particular temperature range, which favors its peak rate of growth and infection potential. These pathogens multiply much more slowly outside a given optimum range.

Bacterial speck (Pseudomonas syringe) and bacterial spot (Xanthomonas campestris) are fungal diseases infecting vegetable crops such as tomatoes and peppers. Bacterial speck usually occurs in early season under cool, moist conditions. As with many fungal diseases, the greatest economical damage to plants occurs at earlier growth stages, but damage often persists until harvest in the form of reduced crop vigor, yield and soluble solids.

Symptoms of the disease first become visible on foliage as dark lesions often initially surrounded by yellowing tissue. Leaf margins where moisture collects tend to be the most affected. Bacterial speck may be confused with salt burn on leaf margins, but it also infects stems, branches and fruit.

Lesions on immature fruit are slightly raised and small, varying in size from tiny flecks to 0.125-inch in diameter and cause raised black spots on mature fruit. Fruit lesions are superficial, seldom penetrating more than a few cells deep. While bacterial speck is more common during the early part of the growing season, the disease can develop on mature plants as well as young seedlings.

According to UC researchers, there are two bacterial speck races in California: Race 0 and Race 1. Many varieties are resistant against Race 0, but none currently possess resistance against Race 1, and the occurrence of this race is increasing throughout the state.

The pathogen thrives in cool, rainy weather conditions. According to UC researchers, disease outbreaks usually are the result of a series of rainstorms followed by prolonged overcast days and cool temperatures. Leaf wetness is a key environmental condition favoring the disease. Bacterial speck is spread by splashing water from rain or sprinkler irrigation.

Morning dews, present day after day, are particularly conducive to bacterial speck development. Sprinkler irrigation during these susceptible conditions further intensifies the problem. If possible, it may be helpful to switch to flood irrigation during periods of high vulnerability. Delaying planting also can minimize bacterial speck by avoiding cool, wet conditions that favor disease development.

The onset of hot weather halts the disease, thus the Mediterranean-type conditions in California limit pervasiveness of the disease to some extent. However, the pathogen can survive in the soil, infecting plant debris and seeds to become problematic in subsequent crops. Certain weed species are also hosts for bacterial speck. Rotation with non-host crops is an important cultural practice to limit the impact of bacterial speck from one year to the next. Since the disease can survive in debris of diseased tissue, tillage to incorporate the residue into the soil more thoroughly can also be helpful.

Resistant varieties can help reduce problems with bacterial speck. However, even when speck-resistant varieties are used, you should remain vigilant when disease conditions are favorable. Some strains of bacterial speck are problematic even on resistant varieties. Additionally, other common bacterial pathogens such as bacterial spot warrant continual monitoring.

Application timing is an important factor in controlling bacterial speck. Since the disease is driven largely by weather conditions, forecasting can help determine when applications should be made. Copper applications at early planting are very effective at halting the progress of bacterial speck. It must be applied preventively to prohibit infection. Repeat applications may be required if rainy weather conditions persist.

There is resistance to copper, so it's only partially effective in reducing the impact of susceptible strains. Copper tank-mixed with Mancozeb can increase its effectiveness, although EBDCs are not allowed by some processors. Chemical control is a temporary, but effective, measure to reduce the disease level until the onset of warmer, drier weather, which stops the disease.

Bacterial spot (Xanthomonas campestris) is a disease affecting tomatoes and peppers in California. The disease can delay fruit maturity and decrease yield. The disease occurs most frequently under conditions of high relative humidity, free moisture on plant tissue and temperatures exceeding 68 degrees. Similar to the spread of bacterial speck, spores are spread by splashing water through sprinkler irrigation or rain.

Major sources of infection are contaminated seed and crop debris. Like bacterial speck, bacterial spot may persist on volunteer tomato plants and surfaces of farm equipment. Bacteria enter the plant through openings such as stomates and hydathodes. The disease also can enter the plant through physical wounds caused by mechanical means such as high-pressure sprayers or cultivation.

Bacterial spot infection in pedicels can cause early blossom drop, lost yield and split sets of fruit. Bacterial spot and speck are most readily distinguished by looking at fruit. Bacterial spot on green fruit first appears as small water-soaked spots. These spots become slightly raised and enlarge up to 1/8 to 1/4-inch in diameter. The center becomes irregular, brown and slightly sunken with a rough, scabby surface. Although ripe fruit is not susceptible, lesions are obvious if green fruit is infected.

Bacterial spot can be suppressed with copper treatments if applied before infection occurs and repeated during the susceptible disease period. UC recommendations are to avoid sprinkler irrigation and make sure there are no cull piles near greenhouses or field operations. Rotating with non-host crops is another cultural practice that can limit the spread of bacterial spot from year to year.

Grading standards for crops such as tomatoes, peppers and tree fruit determine economic damage thresholds and must be taken into account when planning an overall disease management program. For example, since external appearance is important, economic thresholds for fresh market tomatoes tend to be lower than those for canning tomatoes. Such standards are also subject to change.

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