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Symptoms and Signs

Black shank affects tobacco plants at all growth stages. Disease begins on young seedlings or transplants (Figure 2) once soil temperatures rise above 20°C. The most common symptom of the disease is a root and crown rot (Figure 3), but the pathogen may also infect leaves if they come in contact with infested soil during rainy periods (Figure 4). Signs of the pathogen are infrequently observed on plant stems and around leaf lesions, but hyphae often are readily observed in pith tissues upon splitting of the stem (Figure 5). Since other fungi, especially species of Fusarium, are often present in these necrotic pith tissues, microscopic observation of pith cells may help confirm the presence of characteristic hyphae of Phytophthora (Figure 6). In the field, diseased plants are often associated with wet soil and losses may reach 100% in susceptible cultivars in years favorable for disease development (Figure 7).

Figure 2	Figure 3
Figure 4	Figure 5
Figure 6	Figure 7

Seedlings

Once infected, disease develops rapidly in young, succulent seedlings (Figure 8). In the field, symptoms begin as wilting, followed by yellowing of leaves, development of stem lesions, and plant death (Figures 2, 9). Depending on the size of the seedling, the plant may wilt and die within a few days or a few weeks. Infection of seedlings may occur directly on the stem as well as on the roots.

Figure 8

Figure 9

Roots

Root tips and wounds are the primary sites of infection by the pathogen (Figures 10, 11). Infected roots are initially water-soaked, then rapidly become necrotic (Figure 12). On susceptible varieties, lesion expansion progresses rapidly into larger roots until most or all of the root system is destroyed (Figures 3, 13). Colonization of root tissues in susceptible varieties proceeds rapidly and ultimately reaches the stem, resulting in the characteristic black shank symptom (Figure 14) and plant death.

Figure 10	Figure 11
Figure 12	Figure 13
Figure 14

Stems

Stem lesions may continue up the stem 30 cm or more above the soil surface. A longitudinal cut through the stem reveals necrotic pith that is often separated into disks (Figure 15). The disking of the pith does not extend beyond the necrosis observed on the outside of the stem (Figure 16). Disking of the pith should not be used as a sole diagnostic feature of black shank, as other pathogens (Figure 17) and lightning injury (Figure 18) may also cause disking of the pith. The disking caused by lightning injury is not associated with necrosis initially and may extend the entire length of the stem.

Figure 15	Figure 16
Figure 17	Figure 18
Figure 19

In addition to stem symptoms that develop as a result of root infection, infection also can occur directly on the stem. Infection of the stem occurs in old leaf traces that are buried during cultivation, which results in a stem lesion with little or no root necrosis (Figure 19). This symptom is sometimes referred to as stem black shank and may also result in plant death.

Leaves

Above ground symptoms of the stem and root rot phases of the disease begin as a temporary wilt that progresses into chlorosis of leaves and permanent wilting. The extent of chlorosis varies with the level of host resistance and the type of tobacco, with burley tobacco characteristically becoming a brighter yellow than flue-cured plants as the disease develops (Figures 1, 20).

Figure 20

Figure 21

Direct infection of leaves also may occur. Leaf infections may result from zoospores splashing onto the leaf surface or leaves coming into direct contact with infested soil. Lesions may remain brown to black or develop into circular, yellowish-to-brown spots that may reach up to 8 cm in diameter (Figure 4). The pathogen may grow down the petiole and into the stem (Figure 21), resulting in a stem lesion and plant death.

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