Grapes: proper pruning and thinning can prevent black rot
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---Angelo in Springfield (Delaware County) PA
Every year in the late spring I get beautiful little green grapes. But by mid-summer they're all dry and dark looking. I've tried all kinds of fungicide sprays, but none helped. I showed the grapes to my local extension office, but they had no idea what it was. I remember Mike mentioning a product called "Surround" in one of his shows. Do you think that it's applicable here?
---Jing, "doing well in Clinton, NJ"
A. Let's answer that last question first. "Surround" is the brand name for a micronized clay spray; it forms a thin film on fruits that protects them against many diseases and insect pests. It seems to work great; it's the only thing I spray on my peaches (to help keep stink bugs off and prevent BROWN rot—the bane of peach growers); and it should work equally well on grapes.
But if I personally had grapes I wanted to spray for disease prevention, I'd first turn to a fungicide whose active ingredient is a very interesting naturally-occurring organism known as Bacillus subtillis. It's very effective at disease prevention; it's specifically EPA registered for use against black rot; and it's approved for certified organic agriculture. It's sold under brand names like Plant Guardian and Serenade.
Now, I especially wanted to include Jing's email here (we had a LOT of similar ones to choose from) because of the follow-up back and forth we had, which is shockingly typical of people with grape vine problems.
Mike: "Do they get good sun and airflow?"
Mike: "Do you prune the vines every winter?"
Jing: 'Yes, but can you explain exactly how to prune them?'
Mike: "And finally, do you remove grape leaves and fruit clusters during the growing season?"
Jing: 'No; and I don't even know what that means.'
Which makes me very disappointed with the response Jing got from that Extension office. As with tree fruits, proper care of grapevines often prevents these kinds of disease problems—but none of that care is intuitive; you have to learn what to do and then do it every year.
…Like removing grape leaves and baby fruit clusters, which is very similar to what I have to do with my peaches (we remove three-quarters of the fruits early in the season, while they're still tiny). The 'problem' with both plants is their natural lushness. Left on their own, they'd quickly get overcrowded and block airflow to the fruits. And both plants also have a strong tendency to produce too many fruits.
So let's take this step by step. First, select the proper location: grape vines are full sun plants; they'll never do well in any kind of shade--or in soil that doesn't drain well. They also can't be crowded by other plants and should never be fed chemical fertilizers or overfed in any way. A nice mulch of compost underneath is all they need.
And what about those specific rules for pruning?
All grapevines need to be pruned pretty dramatically in late winter. But our fruit growing expert, noted author Lee Reich, Ph.D., explains that the exact style depends somewhat on the specific varieties you're growing and a great deal on what trellis system you're using. Grapes are vines that require strong support, and the exact method of winter pruning will have a lot to do with the style of trellising you have in place.
Your local extension office should have Bulletins on proper pruning and trellising, but given the past performance of the office that Ying tried to use, I'd rely on them as a supplement to the excellent advice in Lee Reich's books; he's my guide for organic fruit growing, and I highly recommend the advice in his Taunton Press books "Grow Fruit Naturally" (2012) and "The Pruning Book" (2010).
OK. So, let's say your grapevines are out in the open in full sun, pruned according to their specific variety and your type of trellis, and the growth is wonderful in the Spring. Now what?
Now comes the part where a lot of people fall down. In "Grow Fruit Naturally", Lee notes that "grape arbors are notorious for becoming tangled messes of low-quality, disease-ridden grapes". Preventing this, he explains, is "all about light and air". If you prune properly and have an excellent trellising system, you may not have to do much else to get good grapes; the fruit thinning I've been harping on and hinting about might not even be necessary.
But it takes many years to develop true pruning courage—and sometimes the vines still get too lush. So if you can see that's its crowded, you'll need to remove both grape leaves and whole clusters of baby fruits to prevent disease and assure a good harvest.
Is there some kind of guide as to when and how much to remove? Yes; and it's called personal honesty. You need to be able to look at the vines in June and July with the perspective of "I probably need to remove some leaves and fruit clusters to get more air and light in there." You'll always get a nice harvest with that attitude. Be in denial and you'll get black rot—especially in a wet and cloudy year.