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Disease Management

An effective disease management program for black shank requires an integration of cultural and chemical practices and planting resistant cultivars. The goal is to reduce pathogen populations in the soil that initiate epidemics of black shank. Since initial inoculum level is related to the development and severity of black shank epidemics, practices that reduce initial inoculum may greatly affect yield and quality of a tobacco crop.

Cultural practices

Cultural practices are vital to the long term management of black shank. Effective cultural practices include: planting on raised beds, which prevents or reduces duration of soil saturation required for zoospore movement in the root zone; crop rotation, which removes potential sources of nutrition for reproductive growth of the pathogen; and stalk and root destruction after harvest, which effectively removes the infected root system from the soil and suppresses inoculum buildup. Planting on raised beds (Figure 36) is not a common practice for many tobacco types and production regions, but it is widely used in flue-cured tobacco production.

Strains of P. nicotianae that cause black shank are host-specific, so rotation with any other crop plant is effective in reducing pathogen populations. Preliminary studies, however, indicate that long term survival in the absence of tobacco may also be related to limited colonization of certain weed hosts (Figure 37). The duration of the rotation should be at least two years, but even long rotations of four or more years will not totally eliminate the pathogen from many soils.

Figure 36	Figure 37
Figure 38

Stalk and root destruction is the physical removal of the root system after the final harvest. Plants are plowed up and inverted to expose the root system to the sun. This practice is not used for all tobacco and soil types, but is effective in reducing late-season buildup of the pathogen on roots that remain alive long after leaves are harvested and stalks are cut. Root and stalk destruction also reduces populations of other pests and pathogens, especially nematodes such as root knot, Meloidogyne spp., that can greatly exacerbate black shank severity (Figure 38).

Soil chemistry greatly affects black shank development. The disease is favored by pH values greater than 6.2 and is suppressed at lower pH values. The suppression of the disease at low pH values has been related to the increased activity of aluminum (Al³⁺) that is present at low pH values, and which is highly toxic to Phytophthora spp. Since Al also is toxic to tobacco at pH values below 5.5, acidification of soil to reduce black shank severity, if attempted, should be done carefully so that phytotoxicity associated with high Al or Mn concentrations does not occur. Soil pH values between 5.5 and 6.0 provide favorable growing conditions for tobacco without providing highly conducive conditions for P. nicotianae.

Host Resistance

The most widely used method of control for black shank is planting of resistant varieties (Figure 39). Multiple sources and types of resistance are available to growers. The most widely deployed type is partial resistance. This type of resistance is present at levels that range from low to high. Continuous deployment of a high level of partial resistance results in the selection of more aggressive isolates of the pathogen.

Figure 39

Figure 40

Two single-gene sources of resistance have been incorporated into tobacco cultivars. Both of these genes provide complete resistance to race 0 of P. nicotianae, and no resistance to race 1 of the pathogen. Deployment of these two genes has resulted in race 1 of P. nicotianae becoming the dominant race of the pathogen present in many areas. Identification of races is based on the inoculation of a set of host differentials (Figure 40). Rotation of partial resistance and single-gene resistance is effective in slowing race shifts and prolonging the effectiveness of single gene resistance.

Chemical control

Chemical control can be effective when used in combination with other practices, but it is not highly effective when used in conjunction with a susceptible variety. The options for chemical control are limited to a single chemistry, with either metalaxyl or its near-identical twin mefenoxam used in single or multiple applications. Mefenoxam is twice as active as metalaxyl, but has the same mode of action. Application rates and time of application (preplant, at first cultivation, or four weeks after transplanting) vary with the level and type of host resistance used and the history of disease in the field. The use of soil fumigants such as chloropicrin may reduce pathogen populations, and fumigant nematicides such as 1,3-dichloropropene reduce nematode populations that enhance black shank incidence and severity.

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