July 2014 Plant Disease Update

A common complaint about tomato plants at this time of the year is that they seem to wither from the bottom up. This symptom is most often the result of a fungal disease called Septoria leaf spot. Like most fungal pathogens, Septoria is favored by high humidity, which is present in the dense, lower canopy of a tomato planting. 

Spotting and death of lower leaves of tomato caused by the fungal pathogen Septoria lycopersici
Spotting and death of lower leaves of tomato caused by the fungal pathogen Septoria lycopersici

Symptoms of small, circular, brown leaf spots appear on the lower leaves first and infection progresses up the plant. Spots may become so numerous that they coalesce, causing the whole leaf to turn brown. Thus, the plant seems to “wilt” or turn brown from the bottom up.

Septoria leaf spot on tomato.
Septoria leaf spot on tomato.

Spots may also occur on stems, but fruit are not directly affected. However, leaf loss may be so severe that yield is reduced. Regular application of a registered, preventative fungicide will control the disease. Spacing plants adequately and staking or caging plants to maintain good air circulation also helps to prevent infection. Removing plant debris at the end of the season will reduce overwintering of the pathogen.