No grape growers know nothing about Downy mildew as it’s very harmful, quick-attacking and difficult to control. In recent years, Downy mildew’s resistance to the chemicals has been rising sharply, and its hazard same. The infection ways have been also changing year by year. So it’s really a headache for the grape growers.
The pathogen of Downy mildew belongs to Mastigomycotina, Oomycete, and it’s a obligate parasite. The mycelium spreads among the host cells and extends into the host cells with a nodule sucker to absorb nutrients.
The most common infection way is through leaves. But it also infect other green tender tissues including young shoots, tendrils, petioles, inflorescences, cobs, fruits, etc.
1: leaves’ damage
After the leaves are infected, initially we can find semi-transparent, oily spots with unclear edges on the leaves. Then the small spots may combine and become large lesions, mostly yellow to brown polygons. When the air’s humidity is too high, a white layer of downy mildew will be formed on the back of the lesion. Finally those lesions will turn brownish and the leaves are easy to fall off early.
2: young shoots’ damage
After the young shoots are infected, water-stained spots will be formed. Later those spots will turn brownish and slightly sunken. In wet conditions, the lesions also produce white downy mildew. When the disease is heavy, new shoots will be twisted, stop growth, and even die. Sometimes, tendril, cobs, and petioles can be infected too, and the symptoms are similar to those of young shoots.
3: inflorescences’ damage
The buds turn dark brown after being affected, and the surface is covered with white moldy material causing buds’ wilting and falling-off.
4: young fruits’ infection
After the young fruits are infected, the infected parts will fade and became hard. The white downy mildew layer will grow and the infected fruit immediately fall off. It starts infection from the fruit expanding period and stops infection after fruits’ coloring.
Recently, there have been various degrees of “black granules” in many regions, in part because of infection with downy mildew.
Aspergillus on the fruit, the more obvious feature is the first incidence of fruit stalk and fruit at the junction, and then the fruit brown sag, the phenomenon of more serious grain dropping.
Control of Downy mildew
From recent years’ cases, there are two obvious characteristics - infecting earlier and earlier, more and more harmful! Although there are weather factors, improper management measures are the main reason.
Therefore, we have to adapt sustainable measures which should take cultivating robust tree as first place and do prevention measures in advance.
1: Cleaning the orchard thoroughly
The downy mildew pathogen enters into the soil following with the infected branches, fruits, and leaves’ falling off. Therefore, clearing the orchard is a powerful measure to eliminate the sources of overwintering pathogens.
2: Cultivating robust tree
Reasonable management of water and fertilizers will promote the growth of the trees without being prosperous, and increase the resistance of the trees.
3: Chemicals control
In the rainy season, it is necessary to regulate the control of Downy mildew, that is, to apply fungicides once every 10 days. Generally, protective fungicides are mainly used.
The common protective fungicides are Copper fungicides, Mancozeb, Captan, Cyazofamid, Azoxystrobin, Pyraclostrobin, etc.
Copper fungicides, Mancozeb: Not safe if use before fruit-bagging, easily to rise phytotoxicity;
Captan: Safe, broad spectrum and long-lasting;
Cyazofamid: High activity, long-lasting and low resistance; but with narrow spectrum and poor conductivity;
Azoxystrobin, Pyraclostrobin: Can be only used as preventation and cann’t mix with organic silicon or EC formulations;
Attention: To apply such products in advance and spray uniformly, especially the back side of the leaves; At initial stage, control the disease center, use mainly curative fungicides.
The common curative fungicides include: Metalaxyl, Cymoxanil, Propamocarb, Dimethomorph, Fluopicolide, Chloroisobromine cyanuric acid, etc.
Metalaxyl, Cymoxanil: Quick action, safe, but short lasting period. Suggest to use with protective fungicides;
Propamocarb: Good systemic property, safe, poor activity, should use high concentration to assure the efficiency;
Dimethomorph: High resistance, should use high concentration to assure the efficiency;
Fluopicolide: Quick absorption, resistant to the rainfall and suitable for rainy season;
Chloroisobromine cyanuric acid: High activity, broad spectrum, but short lasting period. Suggest to use in the disease’ outbreak period and combine with other protective fungicides;
Attention: During the application, can apply contact fungicides first to clean the surface’s pathogen, then apply curative fungicides and finally apply protective fungicides to strengthen the effect.
We have good products for Downy mildew’s control including:
Captan 80% WP
Dimethomorph 12%+Pyraclostrobin 6.7% WG
Cyazofamid 10%+Dimethomorph 30% SC
Metiram 55%+Pyraclostrobin 5% WG