What are the causes of grape downy mildew and what are the control techniques?

Grape downy mildew is an obligate parasitic disease and one of the important diseases harming grape production, distributed in major grape-growing regions worldwide. So why does grape downy mildew occur? What are the control techniques?

1. Why does grape downy mildew occur?

Anyone who grows grapes certainly knows that the downy mildew pathogen prefers to spread and become epidemic in low-temperature, high-humidity environments, beginning to occur in May to June, with July to September being the peak period of disease incidence. But it is necessary for us to explain this in more depth:

1. Temperature

When the air temperature reaches 11°C, oospores can germinate in water or moist soil; sporangia can form at 5-27°C and can germinate at 12-30°C; low temperatures at night also favor the germination and invasion of sporangia. These are the foundations of pathogen infection.

2. Humidity

Humidity and rainfall are the two major limiting factors; the germination and invasion of sporangia and zoospores both require rain or dew, with 95%-100% humidity being optimal.

3. Rainfall

The epidemic of the disease is also related to the seasonal distribution of rainfall; for example, if rainfall is abundant and frequent from May to September, the disease will be more severe. If the summer temperature is 22-27°C, with 10 consecutive days of overcast and rainy weather, or a rainstorm every 8-15 days, and air humidity reaches above 95%, a peak of disease incidence will also occur.

4. Fertilization

Improper fertilization, such as excessive or late application of nitrogen fertilizer, will stimulate grapes to produce new shoots, leading to severe disease incidence. The proportion of potassium in grape cell sap is one of the important factors determining disease resistance; grapes with higher calcium content have stronger resistance to downy mildew.

5. Management capability

Grape cultivation density, the orientation of trellis rows and columns, as well as pruning, trimming and other management practices are closely related to the occurrence of downy mildew.

2. What are the control techniques for grape downy mildew?

1. Thorough orchard sanitation

The downy mildew pathogen overwinters in the soil after entering it with diseased branches, diseased fruits and diseased leaves falling to the ground. Doing a good job of orchard sanitation is a powerful measure to eliminate the overwintering source of inoculum.

2. Cultivate robust vines

Rational water and fertilizer management, strengthened management, promoting robust but not excessive vine growth, truly achieving a moderate tree vigor, and striving to improve the disease resistance of the vines.

Key water requirement periods: before bud break, around flowering, veraison, maturation, and soil burial for cold protection.

Key fertilizer requirement periods: bud break stage, fruit enlargement stage, veraison stage, after harvest and before overwintering. Pay attention to the application of organic fertilizer, apply fertilizer according to the nutritional element requirements of grapes, and combine with Sea Elf biostimulant for drenching application, achieving multiple effects such as balanced nutrition, promoting root growth, improving the soil environment, and stimulating plant wildness.

Strengthen management: ensure good orchard drainage, reasonably control the amount of secondary shoots in the summer leaf canopy, prune unnecessary vine leaves near the ground, pay attention to timely shoot thinning and vine tying, and reduce the primary infection of pathogens from aboveground leaves. When diseased leaves and diseased fruits are found, remove them promptly, bury them deep in a centralized manner and disinfect.

3. Chemical control

Currently, the chemicals for controlling downy mildew are the most numerous among all chemicals used for grapes. In control chemicals, a clear distinction must be made between protectants and curative agents.

Protectants: apply the chemical before the crop develops the disease to prevent pathogen infection. When downy mildew has already occurred and the mold layer has already formed, using protectants again will certainly not work.

Curative agents: use chemicals after disease onset to cut off the spread of the pathogen and control further development of the disease. Therefore, using protectants before disease onset and curative agents after disease onset is the basic principle of our chemical selection.