Small grapes this year

Question:

I have a seedless grape vine that I believe is called California red flame. Last year it produced good growth, but the grapes did not mature more than the size of a Q-Tip head. I have had large grapes and large clusters of grapes in years past. Is it the fertilizer?

Answer:

If you fertilized heavily and encouraged a lot of vine growth, that could have contributed to the problem. If the vine was stimulated to grow too vigorously, few and small berries would be produced. However, there are other factors that must also be considered.

Proper pruning is an important factor in causing the berries to develop properly. Vines that are not sufficiently pruned tend to produce a lot of growth but poor berries. If you think this may have been a cause, contact your local NMSU Cooperative Extension Service office and ask for publications on pruning grapes. 

This year, most of New Mexico was dry during the early summer. This could have prevented proper development of the berries on your grape vine. If the skins of the berries hardened before the rains, the berries may not have enlarged properly. Splitting of the skins may have occurred under these conditions. However, if you were irrigating sufficiently, this is probably not the cause.

When I asked Dr. Ron Walser, NMSU Extension Service Urban Small Farm Specialist, about your problem, he suggested that powdery mildew could have also contributed to your problem. The rains late in the summer would have created the proper environment for development of powdery mildew fungus. There are some fungicides labeled for powdery mildew management in grapes if you wish to use them. However, pruning to keep the vines open to good air circulation may be sufficient to minimize the problem in your garden. In drier summers, powdery mildew should not be such a problem, especially if you irrigate only in the early morning and allow several days between irrigations. Frequent irrigation in the evening creates the humidity that favors growth of powdery mildew under the conditions of our cool evenings.

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Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page.

Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu, or at the Desert Blooms Facebook.

Please copy your County Extension Agent and indicate your county of residence when you submit your question!