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13 October

Phoma at threshold level in many fields in eastern counties report Nick Anderson and Pat Turnbull.

Disease symptoms
Circular white/fawn leaf spots occur from late September onwards. These spots bear small black dots or fruiting bodies of the asexual stage (pycnidia). Rarely, severely affected leaves can die prematurely before the winter, but more often leaf spots tend to have a minimal effect on crop growth.

Fungal development
Fungal mycelium grows down from the leaves to the stems where cankers are formed; in the spring the infected stems show pale brown cankers with a dark brown/black margin which crack and succumb to dry rot. Cankers subsequently spread and penetrate through the stem base, until (in severe cases) the collar is girdled, resulting in weakened stems and restricted vascular flow.

Speed of fungal growth from leaf spots to the stem is calculated at 5 mm per day at temperatures of 15ºC. Therefore, if infected plants are small, and if temperatures are high, there will only be a short window to apply an effective fungicide treatment.Phoma effective fungicides should be applied during the leaf spot phase of the disease in autumn, as once the pathogen has grown into the stem fungicides are generally ineffective.

Threshold Level
Crops should be regularly monitored to determine when the spray threshold of 10-20% of plants infected is reached, so sprays can be accurately timed. In high disease risk situations where plants are small or where
infection begins early (September), it is probably better to work on a lower 10% infection threshold.