Causative agent: fungus: Alternaria solani

Disease lifecycle

Brief History

This fungal disease occurs wherever tomatoes are grown and can infect other crops belonging to the tomato family, for instance eggplant and sweet pepper.

If left un-attended the disease can cause severe defoliation. This would result in reduce yields since the plants’ food producing capacity (photosynthesis) would have been greatly reduced.

Symptoms

The disease can occur on various parts of the plant, leaves, fruit and stem.

Leaves

Small dark brown-black round to irregular spots are seen at first.  These spots will develop into larger spots especially if humidity is high.  The spots are seen on the older leaves first and may have a yellow halo.

As the spots get older the center becomes papery brown and a bulls eye pattern is seen.  This is referred to as concentric rings.

If spots are abundant yellowing of the leaves is more extensive.

Defoliation follows heavy infestation and any fruit on the tree may become sunscald since all the shade would have been lost…this condition refers to where the fruit gets white and soft in the area expose primarily to the sun.

 Fruit

Brownish-black spots about 1 inch in diameter are seen on the fruit.  Infection occurs primarily through the calyx or stem attachment of the fruit.  Lesions can cover the entire fruit and concentric rings may also be seen on the older lesions.

The infected areas may become leathery and fruit drop frequently occurs.

Both green and ripe fruit are affected.

 Stem

The disease can affect seedlings leading to damping off.  Small dark sunken spots are a feature of the infection on stems.  Under the right conditions high humidity and warm temperatures, the spots can become quite large and circular to elongated.  Concentric rings are clearly visible when they occur and the centres of the spots are light brown in colour.

If infected seedlings are planted the disease can manifest itself at the area where the stem meets the soil (the collar area). Girdling occurs (girdling-a ring of forms the stem where the infection occurs, exposing the bark).  This is referred to as collar rot.  When this occurs the plant may die, however if it survives production will be extremely low.

Crops affected

Members of the solanaceae family…tomatoes, eggplant and sweet and hot pepper

 Damage

As mentioned before damage is primarily due to defoliation, fruit drop and damping off.

Where defoliation and fruit drop is heavy, reduced yields will result.

Damping off can cause severe reduction in plant stands, i.e. may severely reduce the number of plants in the field.

 Conditions favouring the disease and how disease can be spread

 Control

• Implement proper sanitary practices – removal of debris from previous crop, soil and seed treatment and removal of volunteer crop.
• Rotate crops and avoid planting tomatoes and hot/sweet peppers at the same time or in rotation.
• Fertilize properly to maintain strong vigorous plants.
• Chemical control – disease can be controlled using both contact and systemic fungicides used in rotation e.g. Benlate rotated with Bravo or Amistar rotated with manzate.
• Use treated seed