DOWNY MILDEW
 Description
 Symptoms
 Disease Cycle
 Control

Yield losses occur when inflorescences, clusters, or shoots become infected, and death can result when severely infected vines defoliate early, predisposing them to winter injury. 

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Symptoms:
The disease is characterized by yellowish-green lesions (oilspots) that form on the upper surfaces of leaves (Fig.1) and turn reddish-brown, necrotic, or mottled as they expand.  A cottony mass of fungal mycelium on the underside of leaves (Fig.2 and Fig. 3) gives the lesions a downy white appearance that is also characteristic of the disease. All green parts of the vine that have mature, functioning stomata, including fruit, leaves, and young shoots, can become infected and covered with a white, downy, sporulating mass of mycelium.  . Infections of young berries can be mistaken for powdery mildew.  When cluster infections occur late in the season, fruit does not soften and appears mottled and light green to red in color. Severely infected leaves may fall prematurely.

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Disease cycle:
The fungal pathogen, Plasmopara viticola, overwinters as oospores in leaf debris on the vineyard floor and as mycelium in buds and leaves. At about 10-inches shoot growth, the fungus becomes active during rainy periods, producing infective zoospores that splash to the undersides of leaves, encyst, and form germ tubes that invade the stomata (tiny leaf pores) when temperatures reach 52 F.  Seven to 10 days after infection, yellowish-green lesions form on the upper leaf surfaces. During the evening, when humidity is >95%, sporulating structures produce sporangia that are disseminated by wind and rain to susceptible tissue.  The sporangia liberate zoospores that can initiate secondary infections. Epidemics develop through secondary spread of the fungus, which is most severe during periods when warm humid nights are followed by rain the next day. In the southeastern US the disease is usually not a problem during the warm summer months, but can become a significant problem in the fall, especially if it is wet. 

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Control:
Cultural - Disease is favored by increased moisture in the air, the soil, and on the vine.  Ensure that soils are well drained, and use good canopy management practices to open the vine canopy to air and light to reduce the amount of trapped moisture and shorten the duration of wetting periods.  Fallen leaves and vine debris that harbor overwintering inoculum should be disked into the soil or removed from the vineyard.

Chemical - Refer to the Winegrape Spray Program in the North Carolina Agricultural Chemicals Manual for current recommendations for downy mildew control (http://ipm.ncsu.edu/agchem/chptr7/706.PDF ). Primary infections can occur from 2-3 weeks before bloom until fruit set, and fungicides are most critical during this time, particularly in problem vineyards. Fungicides should be applied either before infection conditions occur (protectant fungicide) or within 5 days after a potential infection event (eradicant fungicides).  A good guide to monitor for conditions that favor primary infection is the "10:10:24 rule of thumb".  According to the rule, favorable conditions for infection occur after 10 mm (approximately ¼-inch) of rain have fallen while temperatures are 10 C (50 F) or more over a 24 hr period.  In order for infection to occur during the 10:10:24 event the soil must have been wet for 16 hours, followed by rain, and then 2 to 3 hours of leaf wetting.  Oilspots, ifalready present, can sporulate  when  4 hours of darkness occurs accompanied by  temperatures of 13 C (55 F) or more and humidity that exceeds 98%.  Spores produced on oilspots and washed or blown to new leaves can initiate secondary infections in as few as 2 hours if the leaves are wet.  

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