Vinews

No. 14 — July 1, 2019

IA 20x Hand Lens Can Save You A Lot Of Misfortune

There is no doubt that having two grape diseases with mildew in their name can be confusing. Downy and powdery have the same last name mildew. Adding to the confusion are the fungicide control options with some products controlling only powdery or downy mildew.

Identifying which particular mildew you have especially during an outbreak is important because the fungicides used to control mildews are different. Potassium bicarbonate fungicides have activity only on powdery mildew. These fungicides are often applied to “burn out” powdery mildew colonies that are sporulating. The Potassium bicarbonate fungicides are not protective and will not protect tissue from powdery mildew infections.

Phosphorous acid fungicides have activity on downy mildew. The phosphorous acid fungicides are often applied when an infection period has occurred or when active sporulating colonies have been observed. When phosphorous acid fungicides are used as a curative, the higher label rate should be applied.

If infected tissue symptomology does not help you identify which mildew your vines are experiencing then resort to the 20x hand lens and observe the fruiting structures of the colonies.

Downy mildew fruiting structures appear as branched trees with round globes attached to the branches. The globes are termed sporangia and the branched tree structure is a sporangiophore.

Powdery mildew fruiting structures appear as small little barrels that are stacked on top on each other and supported by a single branch. The barrels are termed conidia and the single branch supporting the barrels is a conidiophore.

close-up of front side of leaf with powdery mildew spots, and  an photo B's inset of the conidia of the powdery mildew close-up of back side of leaf with powdery mildew spots
close-up of the sporangia of downy mildew
Powdery mildew on abaxial leaf surface (A), Conidia of powdery mildew (B), Downy mildew on abaxial leaf sur-face (C) (note yellow “oil spots”) and, sporangia of downy mildew (D).

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