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Disease Threats as Missouri Grape Harvest Approaches

As harvest approaches for Missouri grape growers, it’s important to remain vigilant to the end regarding disease management and poor weather conditions can have a significant impact on disease pressure and crop quality this time of the year.  In order for Downy mildew to infect grape tissue the tissue must be wet or have free-water available. In the absence of rainfall, grape tissue may become wet from cool morning dews. The wet tissue provides the medium for the swimming spores or zoospores to infect grape tissue. The optimum temperatures for disease development is 64-76 degrees F.

Once infected secondary infections can spread very quickly and defoliate the grapevines. As a general rule of thumb when scouting, if two sporulating colonies of downy mildew or found on two of fifty leaves scouted the vineyard should receive a fungicide treatment.

Once downy mildew begins to sporulate on the undersides of leaves, the more difficult it becomes to control. Although most all the fungicides used to manage downy mildew should be considered preventative, some fungicides have curative, eradicative, and have antisporulant activity.

Phosphorous acid fungicides applied during an outbreak of downy mildew helps reduce sporulation by burning out active downy mildew colonies. Tank mixing of phosphorous acid fungicides with a protective fungicide such as Captan often results in bringing the pathogen under control. After achieving control of downy mildew, the vineyard needs to be scouted on a short rotation of every 3 to 4 days to make sure lingering downy mildew colonies are not present.

Be forewarned that the downy mildew season extends into the cool fall months of September and October. During these months, downy mildew infections can explode, especially if temperatures nighttime and morning temperatures range between 6476° F, cloudy conditions persist, and moisture from rainfall or heavy dew occur.

Powdery mildew

Unlike downy mildew, powdery mildew does not need wet tissue in order to cause infection. However once grape tissue is infected, powdery mildew thrives in high humidity. Therefore, when scouting focus on evaluating leaves within the canopy interior where the microclimate will be higher in humidity than fully sunexposed exterior leaves. UVlight is also detrimental to powdery mildew providing another reason to scout leaves within the canopy interior.

Management of powdery mildew depends on the level of infection. Light infections can be managed with potassium bicarbonate fungicides followed by a protective fungicide. Moderate to heavy infections are more difficult to manage. I have previously provided management recommendations for moderate to heavy powdery mildew infections and that information is available here.

Sour Rot

Do not become complacent based on the past weather conditions we have experienced this summer. Remember that sour rot initiates when the integrity of the berry skin is damaged. The most prevalent damage to berry skins so far this season is from animal damage. Animal damage is often brought on by the periods of flash drought. Most areas of Missouri have experienced some period of limited rainfall this season. The southern one third of Missouri experiencing the most severe drought conditions.

Your best management of Sour rot is be proactive and apply Oxidate and an insecticide to control fruit flies prior to Sour rot symptom development. Sour rot prone cultivars should be monitored for brix and the first pesticide application being applied at or near 15 brix. For more details on managing Sour rot see the July 18, 2022 Grape IPM Report.

Sour rot Research out of Cornell suggests the following.

  • Start monitoring your vineyards for Sour rot once the grapes attain 12 brix

  • Once berries attain 15 brix and wet rainy conditions persist coupled with warm temperatures this should be a red flag that sour rot is likely.

  • Warm evening temperatures between 6877° F coupled with rainy conditions when berries attain 15 brix should also be cause for alarm

  • Sour rot initiates on wounded berries in which the berry skin has been damaged. Although some evidence suggests that berry wounding occurs in tight clustered cultivars such as Vignoles simply due to the cluster architecture. There are other entries for Sour rot such as hail damaged berries, grape berry moth larvae, or mechanical damage to berries from mechanized leaf thinning, canopy management etc.

  • The application of Oxidate plus an insecticide staring at 15 brix followed by a second application prior to harvest is as effective in controlling Sour rot as compared to four applications.

  • Starting a Sour rot management program after symptoms appear is not as effective as starting a preemptive Sour rot management program prior to Sour rot symptom development.

  • The two spray program worked effectively during a low incidence year (dry season) and a high incidence year (wet season) as compared to a four application spray program.

    Take home: Monitoring the fruit brix, being aware of the weather conditions and timing the spray application, and are key to Sour rot management. Being preemptive can help reduce Sour rot and save you dollars on pesticide applications. — By Dean Volenberg, University of Missouri Grape & Wine Institute

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