Sclerotinia: Positive petal test may not mean "spray"

Small amounts of spores can lead to yield-robbing levels of sclerotinia in continued moist conditions. A petal test to confirm the presence of sclerotinia DNA on petals could be used to provide an indication of pathogen pressure at the time of petal collection.

A test that comes back zero for spores would be helpful in a decision NOT to spray. This suggests that spores needed for infection are not present on the petals at the time of the test.

Spore release and the level of petal infestation can change during flowering. The results of the petal test will only apply to the time during which petals were collected. If you get a dry spell and a low or zero DNA test early in bloom stage, followed by high moisture and humidity, risk from late onset of disease could still be high. Or high test results and high risk conditions at early flowering followed by weeks of hot dry weather could result in very low actual disease infection and yield loss.

The petal test offers a tool for estimating the level of pathogen during flowering. Results will apply only to the period when petals were collected. As mentioned, the level of petal infestation can change throughout the flowering period as environmental conditions change.

Petal infestation levels only account for one aspect of the disease triangle, the pathogen. As a result, this tool should be used with other risk assessment tools available such as the sclerotinia stem rot checklist. Monitoring the environmental conditions and microclimatic conditions is also important for assessing disease risk and monitoring changes in risk through the flowering period.

With current technology, a petal test done at 10-20% flower could be returned within a day or two. Check with your fungicide retailer or agronomist for companies that offer petal tests.

Further reading:
Sclerotinia stem rot management