Black rot is a common term referring to two separate diseases, one fungal, one bacterial. The black rot caused by the fungus Guignardia bidewelli can be especially devastating to grapes and apples, but can strike other plants as well. The black rot caused by the bacterium Xanthomas campestris pv campestris causes heavy losses to the Crucifer family of cabbages, broccoli and related vegetables.
Curing Fungal Black Rots
There is no one-size-fits all solution for curing fungal black rot and the most effective fungicides are restricted to commercial growers. Fungicides suitable for home gardeners are limited, especially if you live in California. The two most frequently recommended by extension services are Mancozeb and Captan.
Curing Bacterial Black Rot
There are no satisfactory chemical cures available to control bacterial black rot on crucifers. The best cure is an aggressive defense. There are species available that resist this disease and they should be the crucifers of choice. Keep your garden free of weeds and debris.
Plant Targets of Fungal Black Rot
Black rot is most often associated with grapes (Vitis vinifera) but it also attacks apple trees (Malus domestica) and other fruit trees, including peaches (Prunus persica), nectarines (Prunus persica var. Nectarina), Japanese plums (Prunus salicina), European plums (Prunus domestica) and sweet cherries (Prunus avium). It can also infect numerous ornamental plants and flowers.
Grapes grow in U.S. Department of Agriculture plant hardiness zones 5 through 8. Most apples will grow in USDA plant hardiness zones 5 through 7, but there are cold hardy cultivars that will grow in USDA zones 2 through 4 and warm weather cultivars that will grow in USDA zones 8 through 10. Peaches grow in USDA zones 6 through 8, nectarines in USDA zones 6 through 8, Japanese plums in USDA zones 5 through 8, European plums in USDA zones 4 through 9 and sweet cherries in USDA zones 5 through 7.
Fungal Black Rot Symptoms
Black rot appears as small yellow spots on grape leaves. As they get larger a black border develops around the spots that become reddish brown in the middle. Tiny black dots show when the spots reach 1/8 to 1/4 inch wide. The fungus may also show as sunken, purple to black lesions on shoots, cluster stems and tendrils of grape vines and as small brown spots on grapes that decays and sinks, rotting the entire grape.
Black rot appears as purple specks on the tops of apple leaves. They grow from 1/8 to 1/4 inch wide, developing tan to brown centers. These are said to be “frog eye” spots because of their appearance. On apples, the spots are purple with a red ring, turning black with a red halo and turning the flesh leathery.
Several kinds of fungi cause spots on plants. If you suspect that you have black rot on a plant species other than grapes or apples, look for spots that have concentric coloring that resemble targets. They typically begin as beige, yellow or brown and turn black as they mature and decay sets in, hence the term black rot.
Fungus Black Rot Cycle
The fungus spends the winter on mummies, shrunken, rotten fruit caused by previous infections. Whether they still cling to the tree or have fallen to the ground, fruit mummies release fungal spores in spring where they are carried by wind and splashing rain to infect new spring growth. Unfolding young leaves are most susceptible to infection from the spores and typically show signs of infection in one or two weeks.
Plant Targets of Bacterial Black Rot
Bacterial rot strikes members of the Brassica genus, especially these cool season annual species:
- Cabbage (Brassica oleracea var. capitata)
- Cauliflower (Brassica oleracea var botrytis)
- Kale (Brassica oleracea var sabellica)
It may also be found on these species.
- Brussels sprout (Brassica oleracea var sabellica)
- Chinese cabbage (Brassica rapa subsp. pekinensis)
- Collard (Brassica oleracae var acephala)
- Broccoli (Brassica oleracea var. italica)
- Rutabaga (Brassica napobrassica)
- Turnip (Brassica rapa subsp. rapa)
Bacterial Black Rot Symptoms
Bacterial rot is easier to identify than fungal rot. Black rot on crucifers begins as a yellow V on the edges of leaves with the point of the V aimed at a vein. The yellow V-shapes grow larger until they hit the veins which turn brown or black. The rot may move down through the leaf stems, then down and up the main plant stems. As it spreads, it opens the way for secondary rotting bacteria that quickly turn the plant into a rotten mess.
Bacterial Black Rot Cycle
Bacterial black rot thrives in warm, wet weather. Heavy fog, rain or dew and temperatures between 75 and 95 degrees Fahrenheit favor its growth. The bacterial spores spread by wind and splashing water. They typically enter leaves at night when leaves draw water from dew or fog. The bacteria can enter leaves for up to eight to 10 hours after landing on them and the plant will show symptoms five to 15 hours later. There are Brassica species available that resist entrance by black rot bacteria.
Fungicides for Fungal Black Rot
Most garden supply centers will carry one of two fungicides recommended for controlling fungal black rot. Mancozeb and Captan are both broad spectrum fungicides, meaning they will kill a wide range of fungi that may infect plants, not just black rot.
Black rot infects new spring growth on grapes and other susceptible plants, so that is what you want to spray. You need to repeat applications throughout the growth period.
Mix 3 to 4 teaspoons of Mancozeb in 1 gallon of water.
Pour the mixture into a backpack tank and agitate.
Spray on new spring growth, covering well, when shoots are 1/2, 1 1/2, 3 and 5 inches long, then every seven to 10 days until grapes are set up to 66 days before harvest.
The application times for Captan are similar to Mancozeb, concentrating on treating new spring growth. Captan is restricted to commercial growers in California.
Captan Application on Grape Vines
Mix 1 to 1 1/2 tablespoons of Captan per gallon of water to spray on grapes.
Spray new spring shoots, covering them thoroughly, when they are 1/2 to 1 inch long.
Spray new spring shoots, covering thoroughly, when they are 3 to 5 inches long
Spray new spring shoots, covering thoroughly, when they are 9 to 12 inches long.
Spray the shoots just before blooms appear. You'll see the buds beginning to open.
Spray shoots again after blooms appear.
Spray shoots every 10 to 14 days until grape harvest.
Captan Application on Fruit Trees
Spray on apple or other fruit tree foliage, covering thoroughly, just before they bloom. You'll see the buds opening up.
Spray on foliage, covering thoroughly, when they are blooming.
Spray on foliage, covering thoroughly, just after the petals fall off.
Spray on foliage and young apples or other fruit every 10 to 14 days until apple harvest.