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Disease Management

Cultural practices.
In many cases, employing sound cultural practices that maintain tomato and potato plants in good health will keep early blight losses below economic levels. Because the pathogen overwinters on infected crop debris, in-field sanitation practices that reduce initial inoculum in subsequent crops are beneficial. Consideration should be given to removing infected material such as decaying vines and fruits from the vicinity of production fields. Control of volunteers and weeds, such as nightshade and horsenettle which are susceptible hosts for the disease, prior to planting the new crop will help to reduce the risk of transmission. Ensuring that seed or transplants are pathogen-free and rotating fields to a nonsusceptible crop also will help to reduce build up of soilborne inoculum.

Optimal tuber maturity is the most important factor for control of tuber infection. Tubers harvested before maturing are susceptible to wounding and infection. Tuber infection can be reduced by careful handling during harvest to minimize wounding as well as avoiding harvesting during wet conditions if possible. Tubers should be stored at 10 to 13°C (50 to 55°F), at high relative humidity and with plenty or aeration to promote wound healing which will reduce the amount and severity of tuber infections that develop in storage.

Figure 9

During the season, overhead irrigation schedules should minimize the duration of leaf wetness in the crop (Figure 9). Irrigation in cool, cloudy periods or late in evening when foliage may stay wet for extended periods should be avoided. Selecting fields with good drainage and an absence of natural impediments to airflow over the crop, e.g. rows of trees, will reduce duration of leaf wetness. Maintenance of adequate soil fertility levels is also critical for managing early blight. The disease is often associated with crops suffering from a lack of nitrogen, particularly towards the end of the growing season on older senescing foliage. Management of other diseases such as Verticillium wilt will reduce plant stress and, hence, early blight severity.

Resistant cultivars.
Complete resistance to early blight does not exist in commercial tomato or potato cultivars. Using wild Lycopersicon species which showed a high degree of resistance in breeding programs has led to the release of a number of tomato cultivars with a degree of resistance to early blight. Apparent levels of resistance are often correlated with plant age. Immature tomato and potato plants are relatively resistant to early blight, but, following tuber and fruit initiation, susceptibility increases gradually, and mature plants are very susceptible.

Figure 10

Chemical control.
Fungicides with protectant and curative properties are registered for use against early blight on tomato and potato (Figure 10). The less expensive protectant fungicides such as mancozeb and chlorothalonil are the foundation of most early blight management programs. These fungicides must be applied every 7-10 days to provide protection of new growth as well as to counter the effects of weathering which progressively removes the chemical from the leaf surface. Advantages of these types of products include their reliable efficacy and multi-site mode-of-action, which reduces the risk of resistant isolates developing in the pathogen population. Disadvantages include the need to apply them regularly and their relatively high use rates.

A newer product available for control of early blight is azoxystrobin. This molecule is a member of the strobilurin (Q_oI) class of fungicides which are highly active against a wide range of fungi. Azoxystrobin is readily taken up into the plant tissue and works both preventively to stop infection and also curatively/systemically to control existing infections. Use rates are considerably lower than the traditional protectant products although cost per acre is typically higher. Due to its single-site mode-of-action, azoxystrobin is at higher risk for development of resistant isolates in the pathogen population. As such, it must be alternated or tank-mixed with products of differing modes of action as part of a typical season-long fungicide spray program. No more than six applications of this fungicide class may be made in a single season. The Fungicide Resistance Action Committee (FRAC) discourages the use of strobilurins in a curative manner. Timing of fungicide sprays relative to environmental conditions and subsequent potential for disease development is critical if good control is to be attained. Use of disease forecasting programs such as FAST (forecasting Alternaria solani in tomatoes) in tomatoes and P-DAY ("physiological" days) in potatoes to correctly time application of sprays and thorough scouting of fields increases efficacy of the fungicide products as well as helps to reduce unnecessary applications and costs. (Figures 11 and 12).


Figure 11	Figure 12

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