I’m a homeowner in Phoenix. This is about a problem with my third year grapevine crop. I have a few Thompson seedless vines, and last year they were very healthy and produced some good grapes.
This year I pruned them as usual in February and trimmed occasionally during the spring. They were doing well, with some leaf tearing/breaking that I thought was caused by wind damage in April and May. Recently yellow brown patches started showing on the leaves, and the grape bunches that were developing started to discolor and shrivel up. The damaged leaves dry up and fall off. This year’s growth on some of the vines shows darkened patches, but some vines continue to produce vigorous new leaf growth beyond the darkened patches. I have cut off most of the damaged material. I water them on a drip same as last year (I estimate about 1/2 gallon each day each plant).
I don't think I can save this years crop, but I want to make sure that I don't lose the plants for next year.
1. Has this problem been seen/reported before? If so, what is it called?
2. How can this condition be treated right now?
3. Should I remove ALL of this year's vine new growth now to prevent damage to next year's crop?
I can provide photos if needed.

Answer #3 · Gardenality.com's Answer · The symptoms could be weather or soil moisture related and there are several diseases and even parasitic nematodes that can cause loss of vigor or decline of grape vines. Some of these can only be positively identified in laboratory analysis of soils. I would suggest contacting your local Extension service or maybe finding and speaking with a professional at a vineyard somewhere in Arizona. If you do, let us know the results and any remedies that are offered.

Brent)

Answer #1 · Gardenality.com's Answer · Hi Ron,

The problem might be Phomopsis cane and leaf spot, caused by Phomopsis viticola. This first appears as spots on leaf veins three or four weeks following rain. Spots are small and dark with yellow margins. Heavily infected leaves are distorted or stunted. Small spots also appear on the bottom portion of the stem and eventually become scabby-looking. Berries may shrivel and canes suffer from bleaching or blackening. To control this pathogen, remove and destroy infected plants, apply liquid lime sulfur in the fall and apply a foliar fungicide in spring if rainfall is anticipated after buds break out.

If you can send pictures it may help us to better diagnose the problem.

Brooks Wilson)

Answer #2 · Ron Webb's Answer · Hi Brooks,
Thanks for your quick and detailed response.
1. Your answer said that Phomopsis viticola appears 3 to 4 weeks following rain. It has been a very dry spring here in Phoenix. I can't remember when it last rained. I think it was a shower in early May. Does this problem normally appear after lots of rain? Maybe too much drip irrigation water?
2.The spots started appearing about a week ago. I have cut away most of the damaged foliage and sprayed with a fungicide called Green Cure (Active ingredient: Potassium Bicarbonate powder) It seems to have significantly slowed the progression of the damage, but it has not completely stopped it. You said to control it I need to destroy infected plants. Do you mean I should remove just the infected areas (this years growth) or remove the whole plant including the 3 year old trunks? I want to save them and have a good crop next year if possible.
I have included photos,
Thanks again for your input.)