Problem Solving

Diseases that cause drying of leaves in tomatoes

The tomato may have been attacked by Bacterial wilt. This  is one of the major diseases of tomato and other solanaceous plants. The disease is known to occur in the wet tropics, sub-tropics and some temperate regions of the world. The disease is caused by the bacterium Ralstonia solanacearum, previously known as Pseudomonas solanacearum. It is one of the most damaging plant pathogens. At the early stages of disease, the first visible symptoms of bacterial wilt are usually seen on the foliage of plants. These symptoms consist of wilting of the youngest leaves at the ends of the branches during the hottest part of the day. At this stage, only one or half a leaflet may wilt, and plants may appear to recover at night, when the temperatures are cooler. As the disease develops under favorable conditions, the entire plant may wilt quickly and desiccate although dried leaves remain green, leading to general wilting and yellowing of foliage and eventually plant death. Another common symptom that can be associated with bacterial wilt in the field is stunting of plants . These symptoms may appear at any stage of plant growth, although in the field it is common for healthy-appearing plants to suddenly wilt when fruits are rapidly expanding. In young tomato stems, infected vascular bundles may become visible as long, narrow, dark brown streaks. A common sign of bacterial wilt of tomato observed at the surface of freshly-cut sections from severely infected stems is a sticky, milkywhite exudate, which indicates the presence of dense masses of bacterial cells in infected vascular bundles, and particularly in the xylem.

Control

Bacterial wilt of tomato is difficult to control but the disease can be controlled  through crop rotation and roguing of infested plants and removal of plant debris.

Best strategy for control of bacterial wilt in the field consists primarily of phytosanitation and cultural practices. In the regions where the disease is endemic, these methods have proven to be effective in some conditions, and should be used:

Before plantation:

Consider an effective weed control in and around tomato fields and aquatic weed control around irrigation ponds.

 Apply 3-4 years rotation and cover crops for infested fields to reduce R. solanacearum, weeds and nematodes.

Do not irrigate rotation and cover crops with R. solanacearum contaminated pond or surface water to avoid reinfestation.

Use well drained and leveled fields and do not use low-lying areas of the field.

 Raise soil pH to 7.5-7.6 and increase available calcium (liming).

Consider using infested fields (after 3-4 years rotation) during cooler months for tomato production (i.e., May to July in Malawi)

During production

 Exclude the pathogen by applying strict sanitation practices (pathogen free irrigation water, transplants, stakes, tools ,etc.).

Continue an effective weed control in and around tomato fields and irrigation ponds.

Irrigate based on water need, avoid over irrigation.

After harvest

Plow under crop residue immediately.

NB:  Other  diseases that cause problems tomatoes include  Early blight (Alternaria solani) and late Blight (Phytopthora infestants). Blights can be controlled by spraying Mancozeb (Dithane M45) 80 WP at the rate of 28g in 14 litres of water for knapsack sprayer, or Chlorothanil (Daconil 2787 W- 75) 40g in 14 litres of water for knapsack or 40g in 1 litre of water for UVL sprayer. Spray at weekly intervals.