In the spring, the grape powdery mildew fungus (Erysiphe necator) releases ascospores from cleistothecia (overwintering structures) that are lodged in cracks in the grapevine bark. When the cleistothecia are sufficiently wetted, ascospores are discharged within four to eight hours and are carried by wind to susceptible plant tissues. This year, a significant portion of ascospores may have been released during warm weather before bud break. Without green tissue present, these spores were lost.

While wetting promotes ascospore release, we have detected powdery mildew ascospores in vineyards on most days from bud break through bloom, even on dry days. However, rain events (such as occurred this past weekend) can significantly boost the number of ascospores in the air, especially within three to seven days after the wetting event. Therefore, this may be a good time to protect grapevines from primary powdery mildew infections. However, not all ascospores are successful at causing an infection; when temperatures drop to 40 degrees Fahrenheit or below, ascospores may not germinate or incipient powdery mildew colonies may be killed. In addition, a cold snap can also make grapevines more resistant to infection. With the moderate to warm temperatures and dry periods in the forecast, this season is shaping up to have great powdery mildew potential.

Once the fungus gets established, it does not need water or rain for infection – in fact, heavy rain is detrimental because it washes the spores from the leaves and causes them to burst. The fungus grows as circular colonies on the plant surface and produces secondary spores (conidia), which are windborne and cause new infections. Under optimal conditions, the disease can spread rapidly, as the time from infection to production of conidia can be as short as seven days. Although secondary infections can occur at temperatures from 59 to 90 degrees Fahrenheit, temperatures between 68 and 77 degrees Fahrenheit are optimal for disease development. Temperatures above 95 degrees Fahrenheit inhibit spore germination, and the fungus may be killed at temperatures above 104 degrees Fahrenheit.

Young berries are the most susceptible to infection and most important to protect. Berry age has a marked effect on susceptibility to powdery mildew. Researchers in New York showed that when clusters of ‘Chardonnay,’ ‘Riesling,’ ‘Gewürtztraminer,’ and ‘Pinot noir’ were inoculated from pre-bloom to six weeks post-bloom, only fruit inoculated within two weeks after bloom developed severe powdery mildew. Berries became substantially resistant to infection by three to four weeks after bloom, resulting in diffuse, non-sporulating colonies on berries, and were virtually immune at six to eight weeks after bloom. Therefore, early sprays (from immediate pre-bloom until four weeks after bloom) are more important for preventing powdery mildew on the clusters. This usually coincides with critical sprays for black rot as well.

For wine grapes, control of late (diffuse) infections is also important as these can predispose the grapes Botrytis bunch rot and sour rot later in the season. Due to the large differences in shoot growth, you may need to protect clusters for a longer period than in “normal” years.