Downy mildew may have its day this season
By Brandy Rawnsley
Inland wine regions can on average go eight or more years without seeing a downy mildew outbreak. This time between downy mildew infections often means growers are out of practice identifying the conditions that result in downy mildew primary infections.
Given the current Bureau of Meteorology weather outlook for a wetter end to the year, it is timely to know what conditions favour disease development as this might be the year for downy to occur.
The downy mildew fungus (Plasmopara viticola) is an obligate pathogen, which means it needs green tissue to survive and reproduce. Yet, overwinter, oospores can survive in soil and plant debris. Oospores allow downy to survive in the vineyard until conditions are just right for primary infection.The initial movement of spores to the vine (primary infection) requires 16 hours of soil wetness at or near 10°C. This wetness can be attributed to rainfall, irrigation or low-lying areas under the canopy. Spores are then spread by rain splash on to the vine.
Plasmopara viticola spores require free water and temperatures greater than 8°C for the infection process to begin. Five days later, characteristic oil spots appear on infected leaves. Warm, humid conditions promote white fluffy mycelium on the underside of the leaf and if left unchecked, downy mildew starts spreading through the canopy.
The best defence against downy mildew is the use of a preventative fungicide. The ideal timing is before wet weather events and when downy mildew warnings are forecast.
Copper and mancozeb are protectant fungicides that protect the surface of the leaf. With good coverage, they can provide adequate protection under good conditions for up to 14 days. However, these can be washed off by rain after application and disease protection can be as short as 3 – 5 days.
Fungicides, such as REVUS®, work differently to protectant fungicides. REVUS locks on to the waxy cuticle of the plant whilst also moving quickly into the leaf to protect both the surface and underside of the leaf. This rapid movement into the leaf gives complete rainfastness and delivers long-lasting protection for up to 21 days.
Dr Belinda (Brandy) Rawnsley is a Syngenta Technical Services Lead for Viticulture and Horticulture.