After bloom, if your berry clusters wither and start looking like ancient Egyptian raisins, you probably have Black Rot. Black Rot attacks the fruit clusters first, starting with a small gray/white spot on the berry. The Black Rot spores were in the flower, having been blown or splashed there by wind and rain from a host on the vineyard floor, old tendrils on the trellis wires, or the bark of the trunk and cordons. The evidence of Black Rot presence on the leaves is easily overlooked by the unpracticed eye. Look for brown spots on the leaves, irregularly shaped, with one or more a tiny black pimples (pycnidia) in the center.
Black Rot does not infect grapes that are beginning to ripen. If the clusters begin to shrivel after veraison, the problem is probably Botrytis.
Black Rot is a vineyard sanitation problem. If you have it this year, you may be able to isolate it and produce a partial crop. But, more than likely, you will have to remove the fruit and start over with a cleaner, more Black Rot free vineyard next year.
Black Rot is easily controlled if you apply the proper fungicide on a regular basis. If your vines are growing in an area of high humidity with poor air drainage, you are going to have more problems with Black Rot. If you don't maintain a clean area around the vines and thereby provide all sorts of hosts for Black Rot spores to bide their time in, you are asking for trouble.
Black Rot may be treated with several spray materials. Our favorite is Mancozeb which is marketed under several names. It also prevents Downy Mildew and Phomopsis. However, Mancozeb is not labeled for use on bearing grapevines within 66 days of harvest. [If you grow roses, and spray for Black Spot, Mancozeb is great].
Our second choice is Ferbam (a.k.a. carbamate), and it may be used up to 7 days before harvest.
There are other products that are specific to Black Rot, including the very promising new product Abound which we have not yet tried. None of these materials are a corrective measure; they are just preventatives. If you already have Black Rot, use of these sprays will only reduce the spread of the disease for the balance of the season and help assure a cleaner vineyard the following year. A dormant spray of lime sulfur and regular use in subsequent seasons of materials specific to Black Rot should eliminate it from your vineyard.
Procedure: Remove all of the shriveled fruit from the vineyard. Spray everything; green parts, surviving fruit (if any), trunk, and supports such as stakes, arbor, or pergola. If there is mulch material underneath the vines, remove it from the area. Mulch is an excellent host for Black Rot spores.
Next spring, in addition to normal pruning, clean all tendrils and twine off of trellis wires (or whatever you use for support). Cleaning the trellis is good practice every year. Remove all pruned material from the area. Remove all mulch material from underneath the vine(s). Buy a dormant spray oil such as lime sulfur and spray all woody parts of the vine and the ground before bud swell. DO NOT use lime sulfur after the buds begin to swell.
Broad leaf weeds such as Black Plantain and Dandelions serve as hosts for Black Rot spores. Broad leaves may be killed without damage to grass with 2-4-D amine and with any weed killer if your vineyard is clear cultivated. It is best to carefully spot kill with weed killers to avoid accidental drift damage to green parts of grapevines. Most grape growers are afraid of 2-4-D Amine use after bud swell, but a small hand sprayer or spray bottle held close to a weed should not cause any problems. Spot killing over a few seasons will virtually eliminate the broad leaf weeds from your vineyard and have a very favorable impact on your effort to avoid Black Rot.
Apply seasonal spray material: Mancozeb up to berry set and Ferbam through veraison every week to 10 days (more often in rainy conditions) from the time the shoots are about 4" long.
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