Biology and Epidemiology

 
 - Survival and sources of inoculum

Alternaria alternata has saprophytic potential allowing it to remain in the soil on a very large amount of lignified plant debris resulting from cultivated plants, such as tobacco, and also from  various weeds. The fungus is most commonly reported to overwinter as mycelium, which readily invades dead plant tissues, allowing the fungus to survive for several months.

- Penetration into the plant and invasion

The fungus most often initially contacts the lower leaves via soil/water splash. They affect first the lower leaves. Penetration can occur in two ways: either directly through the cuticle or through stomata (figure 1) or various injuries on the leaves. The first mode seems to be the most common. Penetration is largely favoured by the presence of water on the leaves. Contaminations take place more easily on the underside of the lamina. After penetration, the fungus will gradually invade the leaf tissues. The first symptoms are visible after 5-8 days of incubation.

- Sporulation and dissemination of the fungus

A few days after the appearance of spots and the invasion of necrotic tissue, if weather conditions are favourable, conidiophores bearing conidia (figure 2) are formed on the surface of damaged tissues, which will serve as secondary inoculum. Numerous spores are most commonly spread by wind over long distances or in a more limited way through the splashes of water, for instance on leaves and neighbouring plants.
 
Alternaria alternata also colonises the floral parts and contaminates the seeds, thus they are able to transmit the disease.

- Favourable conditions for the fungus development

Moisture is the factor that most influences the development of Alternaria alternata on tobacco. Indeed, the disease is particularly damaging during wet weather (long periods of rain occurring during harvesting). Conversely, there is little damage in dry season. Dew wetting the leaves for several consecutive nights not only promotes germination and penetration of conidia, but also their production. Ambient temperature contributes less to the development of disease. Its optimum temperature appears to be between 20-25 °C, but disease can develop at temperatures between 16 and 31 °C.

Other factors affecting the susceptibility of plants are commonly reported:
- cultural practices: all cropping errors leading to a slowdown in plant growth and to stress (e.g. hardpan, etc.) make plants more susceptible. It is the same when the plants are not topped and suckered. Conversely, in countries where maleic hydrazine is used as a sucker control agent, the effects of brown spot are reduced;
- fertilisation and soil type: unbalanced fertiliser, with low potassium and nitrogen content weakens plants. The N, P, K ratio also seems to influence the disease which sometimes takes different forms from one soil type to another;
- plant health: simultaneous attacks by other diseases or certain pests seem to modify the sensitivity of plants. In fact, most severe damage by brown spot is found in crops where for instance Fusarium oxysporum f. sp. nicotianae or root-knot nematodes occur.

Virginia type tobaccos are more sensitive to this disease than Burley and dark tobacco.