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Disease Management

Preplanting management options
Cultivars: No potato cultivar is immune to all strains of P. infestans, but some cultivars are more resistant than others (Figure 23). If the climate in which the potatoes are grown is relatively dry, even low levels of resistance may significantly reduce disease severity. Likewise improved host resistance can be combined with timely foliar fungicide sprays to enhance effective disease management.

Figure 23

Figure 24

Site selection: Good drainage and good air movement will help reduce moisture levels in the canopy. Fields bordered by trees (Figure 24) and dense vegetation should be avoided. The shape of the field may affect the ease and frequency of fungicide applications.

Crop rotation: Rotations of two to three years to non-host crops are recommended. Besides potato and tomato, several weeds and ornamental plants in the Solanaceae family are known to be susceptible to late blight. If oospore production becomes widespread, rotation plans may need to be modified to accommodate this new source of inoculum. The pathogen survives in infected tubers which decay relatively quickly, but oospores may survive in soil for many years.

Elimination of overwintering inoculum: In the absence of oospores, tubers infected during the previous season are the most important source of initial inoculum. Surviving tubers may be found in cull piles (Figure 20) and tubers left in the field at harvest. Culled potatoes should be left on the soil surface to freeze, trucked to a landfill, or buried at least 1 m (3 feet) deep. They also may be fed to livestock provided steps are taken to secure them during transport and dump them only on impervious surfaces. Volunteer plants in the spring should be destroyed to minimize initial inoculum.

Figure 25

Planting of pathogen-free tubers: Only certified seed tubers (Figure 25) should be planted. However, currently (2005), even certified seed tubers may be allowed to have up to 1% incidence of late blight. Fungicide treatments are available for protecting freshly cut seed tuber surfaces.

Management in established potato fields
Hilling: Soil can be made deeper around the base of the plants after emergence of the young potato plants. Hilling helps in early weed control and minimizes tuber infections from sporangia that wash off the leaves of infected plants into the soil.

Irrigation: It is important to minimize the time that leaves are wet to help prevent foliar infection (Figure 26). Irrigation should be timed so that length of the night dew period is not extended- i.e., no late afternoon, early evening, or morning irrigation in order to allow plants time for drying. Excessive irrigation can wash some of the "hilled" soil away from the base of the plants, exposing tubers to greater potential infection.

Figure 26

Fertilization: Excessive nitrogen fertilization increases canopy cover, delays maturity and may reduce yield. Delayed maturity results in more foliage exposed to potential infection for a longer time, increasing the risk of late blight.

Scouting for disease: Field scouting alerts growers that the potential for a severe disease outbreak exists. Scout where moisture is likely to persist (low areas, near hedges and trees) and where fungicide applications may be difficult because of obstructions (corners, trees, utility poles). Infected plants should be burned or plowed under if found in an isolated "hot spot." Current season temperature, relative humidity and rainfall data are used to predict disease outbreaks, based on historical patterns. Some growers apply fungicides when the forecast indicates disease development is likely, which may be more risky than routine preventive fungicide applications. Forecasting systems (Figure 27) for late blight include the Hyre system, the Wallin system, and BLITECAST which incorporates both systems. Some commercial microcomputer units are available to provide forecasts.

Figure 27

Figure 28

Fungicide applications: Fungicide applications (Figures 1, 28) are an important means of late blight management, particularly in humid areas. Contact fungicides are effective and have not resulted in pathogen resistance after many years of use. They coat the leaves to prevent infection, but cannot stop infections once they occur. Therefore, they must be applied before plants are exposed to spores. Systemic fungicides can offer some post-infection control.

Most of the newly introduced strains of P. infestans are of special concern because of their resistance to metalaxyl/mefenoxam. In the early 1990s, some growers lost entire potato crops (Figure 29) to them. Special emergency permits were granted for the use of fungicides not yet registered in the U.S. but available in Europe and other areas. Some new systemic fungicides, such as Acrobat (dimethomorph) and Curzate (cymoxanil), were recently registered. In addition, growers have been encouraged to concentrate on the prophylactic use of contact fungicides. Some growers routinely use contact fungicides early in the season, but then rely on forecasting systems after plants mature and the canopy is established.

Figure 29

Figure 30

Management at harvest and in storage
At the time of harvest, it is possible that some late blight infections are present in the foliage. To prevent inoculation of the tubers during harvesting, foliage needs to be destroyed so that no green tissue remains. In the past, vines were killed with flames (Figure 30). Herbicides are currently used to assure that plants have died completely before harvest. Infected tubers should be removed before going into storage, and disposed of properly. Blighted tubers do not generally store well under typical storage conditions.

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