Grape Vine Problems: Mildew, Black Rot, Birds and What to Do About Them

Airflow is the treatment

The fungal diseases that ruin grape crops all need leaf wetness and still air to get going. A vine with shoots thinned to a hand's width apart, laterals pinched, and the leaves immediately around each bunch opened up dries out within an hour of rain. A crowded vine stays wet all morning, and that difference decides whether you have a problem at all.

Sanitation is the second half of it. Most of these fungi overwinter in mummified berries left hanging and in fallen leaves under the vine. Stripping the shriveled bunches off at winter pruning and clearing the debris beneath removes the source of next spring's infection, and costs nothing but ten minutes.

Never compost mummified grapes or diseased leaves in a heap you will spread under fruit. Burn them or send them out with the household waste.

Powdery mildew and downy mildew

These two look superficially alike and are treated differently, so learn to tell them apart. Powdery mildew is a dry-weather disease: it appears as a dusty white or gray coating on the upper leaf surface, on shoots and on the berries themselves, and it does not need rain, only humidity and shade. Infected young berries stop growing, turn hard and split as the healthy tissue around the wound expands.

Downy mildew needs actual wet. It shows first as pale yellowish oil-spot patches on the upper leaf surface, with a white downy growth appearing on the underside directly beneath them in humid weather. Badly hit leaves brown and drop, and a vine that defoliates in midsummer cannot ripen its fruit or build reserves for winter.

For both, act at the first sign rather than waiting. Remove the worst-affected leaves, open up the canopy further, and treat with a sulfur or copper-based fungicide approved for grapes in your area, following the label intervals. Do not apply sulfur in temperatures above 85F (30C) or you will scorch the leaves.

Black rot and bunch rot

Black rot is the disease that turns a promising bunch into a cluster of hard black raisins. It begins as small tan spots on the leaves with a dark margin, then infects the berries just as they begin to size. An infected berry develops a pale spot, browns, shrinks and hardens into a mummy that stays on the vine all winter and infects next year's crop.

It cannot be cured once in the berry, so the whole battle is timing. Remove every mummy at winter pruning, and protect the vine from bud break through to about a month after flowering, which is when the berries are susceptible. After the berries have sized up they become resistant.

Gray mold is the late-season one, arriving with rain during ripening. It enters through split skins and insect punctures, which is why berry thinning to prevent splitting matters so much. There is no rescuing an affected bunch; cut it out and remove it from the garden so the spores do not blow onto the rest.

Insects and animals

Birds and wasps together take more home-grown grapes than every disease combined, and they arrive exactly when the fruit becomes worth eating. Both need physical exclusion; nothing else works reliably.

1. Bag the bunches Slip a paper or fine mesh bag over each bunch as soon as the berries start to color and close it round the stalk. This is the single most effective measure against birds, wasps and late rain splash.
2. Net the whole vine Where there are too many bunches to bag, drape netting with a mesh under half an inch (1 cm) and hold it clear of the fruit on a frame. Netting resting on the fruit lets birds peck straight through it.
3. Remove wasp food early Wasps rarely open an intact skin; they enlarge damage that is already there. Cut out every split or bird-pecked berry as soon as you see it and the wasps largely lose interest.
4. Watch for leaf-rolling caterpillars Leaves spun together with silk usually hide a caterpillar. Squeeze the rolled leaf or pick it off. Damage is cosmetic on a healthy vine unless the caterpillars move into the bunches.
5. Check for scale on old wood Brown limpet-like bumps on two-year and older wood, with sticky honeydew and sooty mold beneath, mean scale insects. Scrub them off in winter with a stiff brush and treat with a winter oil wash.
6. Look under leaves for mites Fine speckling and a bronze cast to the leaves with faint webbing underneath is spider mite, worst in hot dry spells against a wall. A forceful water spray to the leaf undersides several days running knocks the population back.

Problems that are not diseases

A good deal of what looks like disease on a grape vine is a growing fault. Berries that stay tiny and seedless scattered through an otherwise normal bunch are a pollination failure from cold or wet weather at flowering, not an infection. Bunches where the stalk shrivels and the berries dry from the tip back are usually water stress at a critical stage.

Sunken brown patches on the side of the berries facing the sun are scorch, most often caused by stripping leaves off the hot side of the vine too suddenly during ripening. Leaves yellowing between green veins point at a nutrient lock-up on chalky soil rather than at anything you can spray for.

The distinction matters because reaching for a fungicide against a cultural problem wastes time while the real cause carries on. If the damage is even, symmetrical and follows the sun or the water, look at your management before you look for a pathogen.

Keep a record of first sightings

Note the date you first see each problem and where on the vine it appeared. Fungal diseases arrive at strikingly consistent points in the season for a given garden, and two or three years of first-sighting dates tell you when to open the canopy and when protection needs to be in place, well before the symptoms show.

Record what you did and whether it worked, too. It is easy to repeat a treatment for years out of habit, and the record is what shows you that the year you thinned the shoots properly was also the year the mildew never really took hold.