The frequent rains, heavy dews, fog, overcast skies and moderate temperatures have promoted infections by downy and powdery mildew. Strictly protectant fungicides cannot stand up to the heavy rains that have occurred this year. Dense canopies prevent air movement to dry off foliage and fruit and also impair good deposition of sprays.
It is important to make sure you distinguish between downy and powdery mildew. Downy mildew sporulation is very clumpy in texture while powdery mildew looks like a coating of flour or talcum powdery. Most of the time, downy mildew will be on the under surface of leaves while powdery can be on upper or lower surfaces.
Above: Clumpy white sporulation of downy mildew on leaf and fruit.
Above: Dusty white to grey sporulation of powdery mildew on leaf and fruit.
The majority of systemic fungicides are prone to resistance. In order to reduce this risk, the best strategy is to eliminate as many of the spores as possible before applying a resistance-prone product. These treatments cannot reverse the infections that have occurred but they will at least reduce the number of new infections.
If downy mildew is a severe problem, apply one of the phosphorous acid products (Aliette, Confine, Phostrol or Rampart) (in alphabetical order, not effectiveness – they all give the same degree of eradication but do not give a lot of protection) tank mixed with a systemic or locally systemic product to extend protection.
If powdery mildew is a severe problem, apply MilStop/Sirocco, Pure Spray Green Spray oil, Vegol or Cueva to eradicate the superficial growth. Follow this spray with a systemic or locally systemic product to give protection. Do not tank mix MilStop/Sirocco with pH adjusters, oil or products not compatible with mild alkaline solutions. Do not apply Pure Spray Green Spray oil within 14 days of Captan, Maestro, Folpan, Ambush, Perm-Up, Pounce or sulphur products or when temperatures are above 25°C.
In trials done over several years, the cold hardiness of buds was more closely correlated with the severity of foliar disease (powdery and/or downy mildew) than with the amount of lesions on canes.
The good news is that fruit is no longer susceptible 6 weeks after capfall. Any new sporulation that you observe on the fruit is the result of infections that occurred previously. It is still very important to continue to protect the foliage. Healthy leaves are needed to ripen the fruit and contribute sugars to help the vine acclimate for the winter. The rule of thumb is that it takes approx. 1 square meter of leaf surface to ripen 1 kg of fruit. This amounts to 12-15 actively photosynthesizing leaves (not including the basal 4-5 leaves which are not net exporters of sugar by veraison) per cluster. The other thing to consider is that severe infections this year mean more overwintering spores and high disease pressure next year.
Even though the fruit is no longer susceptible to new powdery or downy mildew infections, protecting the leaves, especially with all the infections around, is critical.
There are sporadic sporulating Botrytis infections in some vineyards. These are likely the result of the activation of infections that occurred during bloom (which was very wet) by injuries such as berry squeeze or hail. Generally berries bruised by leaf removal don’t express infections but rather dry up and fall off. Having sporulating berries in the vineyard will increase the pressure for disease. In many years of sour rot research I have observed that bicarbonates (MilStop/Sirocco) seem to dry up berries infected with Botrytis so this may be an option when you are choosing a product to manage powdery mildew in addition to your regular Botrytis sprays.
Sporulating Botrytis infection
Above: Leaf removal injury
Berries injured by leaf removal will turn brown and shrivel within a day of injury. Eventually they will dry up.
Exposure of clusters through leaf removal is also important especially in high disease pressure years, although complete stripping of the fruiting zone is not beneficial from a disease management point of view. If clusters are piled on top of each other, it is beneficial to thin out the clusters to promote cluster drying and better spray penetration. Often Botrytis infections start in the middle of the cluster or in crowded clusters.
Fruit becomes susceptible to direct penetration by Botrytis spores when it reaches 8°Brix so it is important to apply a Botrytis product at veraison and ensure excellent coverage on all sides of the cluster. If the wet weather continues, another fungicide application may be required 2 weeks later if infection continues. Make sure to rotate among chemical families as using them on sporulating infections increases the risk of resistance. We don’t have a large arsenal for Botrytis and cannot afford to lose any of them!
Click here for a pdf of this post: Fungal disease management in grapevines for the remainder of the 2017 season