At Winchester, we did not experience the forecasted rain event yesterday (5/08/09). It was a light shower in this morning with trace amount of precipitation. One thing I noticed was the night time temperature. It was above 68F with relatively high relative humidity (mostly in 8-90% range). Under this condition, downy mildew fungus (it’s not technically a fungus, but I call it fungus for convenience) can produce a structure called sporangiophore which is basically a tree of sporangia which contains spores called zoospores.

Typically, we consider the risk of spore production is high when there are more than 3 consecutive nights with temperature above 55F with high RH. Night time temperature in last few days are (5/6/09: lower 50’s, 5/7/09: lower 60’s, 5/8/09: lower 50’s, and 5/9/09: upper 60’s), and you know the RH was high. The first potential infection event was on 5/4/09 around Winchester, and it takes 7 days (depends on temperature) to produce spores. Even if the 5/4/09 rain was not enough, we had a plenty of rain events after that. Thus, if the trends continues, the conditions for spore production will be met in the near future. I’ll keep updating on this.

But please do not panic. I’m just talking about biology of this fungus to inform you that even though you may not think of it, the life of fungus is moving along with the season. They do not come out from nowhere like a magic. (at least for major fungal diseases for grape) It takes its steps, but we cannot see it. Protective fungicide application will protect your vine from infection, and as I noted several times, the situation is different in each vineyard.