Brown Spots on Grapevine Trunk

(Question)

Dark Brown Spots on TrunkThis spring, I planted a grapevine in a large container on my terrace in Toronto, which faces west and gets full sun from about noon. (I don’t remember the exact variety of vine, but the grapes are seedless green.) The vine did very well all season, growing quickly and putting out lots of leaves. However, about three weeks ago a few of the oldest leaves started turning yellow and falling off. This yellowing has spread up the vine from the oldest to youngest leaves, all of which look a bit sickly now. I’ve also discovered a number of dark brown spots on the trunk. Are these just signs of the end of its growing season, or is there something wrong with it?

(Answer)

Dear Gardener,

Thank you for writing, and hopefully we can get you some help with your grapevine. Terrace gardening can be very rewarding, and from your picture it looks like you’ve taken very good care in supporting your vine’s vigorous growth.

Grapevines need LOTS of light, heat and moist, but very well-drained soil. And so terrace gardening can be a bit of a challenge, drainage-wise. Depending on the size of your pot, (how large is “large”?) and the saucer below, during rainy periods water can collect over extended periods of time, and hence not allow your plant’s soil to drain and breathe properly. How did you plant your grape: with a good layer of drainage gravel, or clay pieces, at the bottom? Further, and through no fault of your own, you had to deal with the exposure that you have, and your grape vine waited until after noon for direct sunlight. Grapes are very susceptible to a broad spectrum of diseases, but fortunately I don’t see any sign of this on the leaves in your photo. Nor do your plant’s leaves show any leaf curl — so by September 22, perhaps the leaves dropping could be deemed as end-of-season natural attrition.

However: you mention your grape “growing quickly and putting out lots of leaves”. And your vine is growing up against a glass pane, preventing a clear passage of air, so it is possible that without some pruning — KEY to growing grapes — throughout the season, that these leaves were dense enough to prevent air and sunlight from penetrating into the centre of your plant, thereby creating a perfect “climate” so loved by fungus.

Hence, what you’re seeing on the stems is quite possibly a fungus: either powdery mildew, Erysiphe necator, or Anthracnose, Elsinoe ampelina  — and defined by Agriculture Canada as:

Brown to black irregular blotches that can measure up to a few centimetres, follow the gradual degeneration of the fungus over the course of the season The spots have indistinct margins and remain visible even following shoot lignification.

And, for your further research, here is their identification guide: http://www.agr.gc.ca/resources/prod/doc/sci/pub/pdf/id_guide_major_diseases_grapes_e.pdf

Also, please see the Toronto Master Gardener articles, below, on the subject of growing and pruning grapes vines:

http://www.torontomastergardeners.ca/askagardener/do-i-need-to-prune-my-12-year-old-grape-vine/

http://www.torontomastergardeners.ca/askagardener/growing-grapes/

It’s unlikely that the fungus could kill your vine, but when your plant is old enough to bear fruit, you want as healthy a plant as possible!  Organic fungicides and insecticides will be the choice for a natural gardener. Sulfur dust is one of the most effective and inexpensive organic fungicides available to home gardeners to control powdery mildew. Your local garden centre, or nursery, should be able to help you, and please follow the package instructions to the “T”.
One final bit of advice: once you have pruned and treated your vine, it is important to remember that old plant material can harbor the fungal spores in soil even over the winter, so it is essential to clean up around the vines after pruning. All the best with your grapevine, and remember that after all your care and effort, your grapes will taste all the sweeter !