How do you identify grape white rot? What are the control techniques?

Grape white rot, commonly known as water rot or bunch rot, mainly damages the fruit clusters (including the rachis, pedicels and berries), and can also damage the canes and leaves. It is a major disease that occurs commonly in grape-producing regions of China. So how do you identify grape white rot? What are the control techniques?

I. How do you identify grape white rot?

1. Disease on the fruit cluster

It often causes large numbers of fruit clusters to rot; clusters close to the ground are prone to the disease. Affected clusters generally show water-soaked, light brown, irregular lesions and become rotten. One week after infection, a dense layer of greyish-white small granules appears on the berry surface, and the diseased part gradually loses water, shrinks and spreads toward the berries. The berry stalk area first turns light brown, then gradually enlarges and becomes soft-rotted, after which the entire berry turns brown and rots, but the berry keeps its shape. The rachis and pedicels often dry up and constrict, and in severe cases the whole cluster rots. Diseased berries hanging on the vine gradually shrivel and dry into mummified fruit with distinct angular ridges. If the fruit becomes diseased before veraison, the diseased berries have very low sugar content and readily lose water and dry out; the dark brown mummified berries often hang on the vine for a long time without dropping and are easily confused with dead-arm disease. If infected after veraison, the diseased berries do not dry out easily, and when shaken, the berries or even the whole cluster drop off very easily.

2. Disease on the canes

It is most prone to occur at injured sites, at the pinching points of new shoots, and at the attachment points of the cluster stalks after harvest, and especially on water sprouts emerging from the soil. At the initial stage, the lesions first appear light yellow and water-soaked, with dark brown edges. As the cane grows, the lesion also expands upward and downward, turns brown, becomes sunken, and is densely covered with greyish-white small granules. Afterwards the epidermis turns brown and lifts, the cortex separates from the xylem, and it often splits longitudinally into a tangled, hemp-like mass. When the diseased cane encircles the cane, the middle becomes constricted, and sometimes at the upper end of the lesion, at the boundary between diseased and healthy tissue, it becomes thickened or knobby because nutrient transport is blocked.

3. Disease on the leaves

It mostly occurs at the leaf margins, leaf tips or wounded areas. At first there are light brown, water-soaked lesions, which gradually spread toward the middle of the leaf and form indistinct concentric rings; after drying, the lesions break up very easily. In humid weather, white small dots (pycnidia) form, mostly distributed near the veins.

The most important characteristic of this disease is that, under humid conditions, the affected parts have a special moldy, rotting smell.

II. What are the control techniques for grape white rot?

1. Choose resistant varieties

Choose disease-resistant varieties according to local conditions; for example, Muscat Hamburg and Longan are relatively resistant to white rot.

2. Do a good job of clearing the vineyard

Do a good job of clearing the vineyard to reduce the primary infection source. Promptly remove diseased fruit, diseased canes, diseased leaves and other diseased tissues. Sweep up fallen twigs and leaves on the ground and burn or deeply bury them in a centralized way to reduce the infection source. Do good drainage work, lower the humidity in the vineyard, and minimize unnecessary wounds to reduce opportunities for pathogen infection. After fruit set, lay plastic film on both sides of the grapevines to prevent the spread of pathogens from the soil and reduce infection opportunities.

3. Chemical control

For soil disinfection, in severely affected vineyards, before the disease occurs, mix 50% thiram powder, sulfur powder and lime powder uniformly in a ratio of 1:1:2, then spread the mixture on the vineyard ground at 1-2 kg per mu, or spray the ground with 200-fold sodium pentachlorophenate, which can reduce the incidence of the disease.

After the vines are taken out of the soil, first spray once with 3-degree lime sulfur mixture, and spray once with Bordeaux mixture before flowering.

When the grapevines sprout 3-5 leaves, begin combining control with that of other diseases and spray once every 7-10 days. Especially after rain, spraying protection should be strengthened. You can spray 10% difenoconazole water-dispersible granules at 2000-3000-fold, or 50% carbendazim wettable powder at 600-fold, 70% thiophanate-methyl wettable powder at 700-fold, or 80% mancozeb wettable powder at 800-fold. When fruit growth has basically stopped, azole pesticides such as myclobutanil, flusilazole, propiconazole and diniconazole can be used. They should be reasonably combined and used alternately with 1:0.7:200 Bordeaux mixture fungicide.

Warm reminder:

Grape white rot can occur throughout the growing season. Large amounts of pathogenic bacteria generally remain in vineyards, and as long as humidity is sufficient, they can infect through wounds and cause disease. Therefore, from spring to autumn is the key period for controlling grape white rot. Storms and hail cause large numbers of wounds on grapevines and provide the humidity needed for grape white rot infection, thus providing better conditions for the occurrence of grape white rot.

The early or late occurrence of white rot is directly related to the early or late rainfall in May and June and to the amount of rainfall. Therefore, after each rainfall (above 5 mm), promptly spray a systemic therapeutic fungicide, and thereafter spray fungicides at certain intervals.

When there are storms, hail and weather that damages grapevines, promptly spray fungicides for control. In summer, the incubation period after white rot infection is generally 5-6 days. Therefore, control must be carried out before the incubation period ends.