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Pith Necrosis

Pith necrosis of tomatoes is commonly caused by the bacteria Pseudomonas corrugata though other Pseudomonas species may be responsible. It usually attacks when plants are grown in humid conditions and are given lots of high nitrogen fertiliser. It is thus often found in greenhouse grown tomatoes. Cool nights and even cloudy days also increase the likelihood of infection so tomatoes are most likely to be infected early in he the season.

How It Spreads

The bacteria lives in the soil so pith necrosis can spread from plant to plant when tomato plants are situated close together or when they're planted in soil that has recently grown tomatoes or other nightshade vegetables. If diseased plant matter is used to mulch the soil or is added to compost that doesn't get hot enough and is then used to enrich the soil of other tomato plants, new plants can easily become infected as well.

It is possible that plants may be infected as seeds.

Plants affected

Tomatoes and other nightshade vegetables are most commonly affected by this disease.

Symptoms

Initially leaves will turn yellow and may wilt. If the infection is serious, black or brown sports may appear on the stems, which also often swell. They may later shrink and crack. The most telling symptom can be seen if you cut open a stem - the centre (also called the pith) may be brown and may have holes in it or in advanced cases, the stem may be hollow. This is where the disease gets its name.

Treatment

As with many bacterial tomato diseases, there is no effective treatment for pith necrosis. Plants may recover on their own however, as the weather warms and becomes sunnier. Potted plants in greenhouses should be moved outside to reduce humidity and improve ventilation.

Prevention

As indicated above, it is unconfirmed but suspected that plants may become infected as seeds. For this reason, do not save seeds from fruit collected from infected plants or from plants that have overcome infection.

To prevent the spread of the bacteria from plant to plant, situate tomatoes with plenty of space between them, rotate your crops (ensuring that tomatoes and other nightshade vegetables are not planted in the same spot for at least four years) and interplant your tomatoes with alliums and other plants that can help kill bacteria. A crop of mustard before or after tomatoes can help clean the soil as well.

To prevent infection in the first place, only water the soil (not the foliage) and provide plenty of ventilation if you're growing tomatoes in a greenhouse. Ensure all diseased plant matter is disposed of and not used as mulch or added to compost (unless you're absolutely certain that the compost is hot enough to kill the bacteria).

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