What to do when you start seeing disease in the vineyard: Post-infection treatments

When fungal diseases show up in vineyards, it is not too late for action. There are various things to consider when making post-infection fungicide applications, including weather conditions and product efficacy.

Unusually humid weather this 2014 season has provided plenty of opportunity for diseases to become a problem in vineyards. Black rot, Phomopsis, anthracnose, powdery mildew, downy mildew and Botrytis have all been seen to varying degrees in Michigan vineyards this year. The general approach to disease management is to apply preventative fungicide sprays and careful and timely canopy management. Field scouting for diseases is an important component of an integrated disease management approach. However, sometimes diseases can take us by surprise and symptoms appear, either due to high disease pressure, poor fungicide timing, suboptimal spray coverage, fungicide wash-off due to rain, or a combination of factors. In addition, fungicide resistance may play a role in some cases, such as in grape powdery mildew.                                                   

Michigan State University Extension advises growers to prepare to apply post-infection treatments if necessary. Several fungicides have post-infection activity, like the sterol inhibitors such as Elite and Rally, which means that they’ll stop disease development if applied after an infection period has occurred, but before disease symptoms appear. Their activity usually ranges from 24-72 hours after infection for black rot and powdery mildew. The strobilurins have less post-infection activity, but have fairly strong anti-sporulant properties, which can help reduce sporulation and disease spread. However, very few fungicides have the ability to eradicate active infections once symptoms have become apparent. At most you can expect to knock them down a bit and suppress sporulation while you keep infections from spreading to healthy clusters and leaves.

For visible powdery mildew, oils such as Tritek and JMS Stylet Oil are the best in our experience – two sprays (1-2 percent v/v) five to seven days apart are needed to observe a substantial reduction in disease. Apply as soon as you start to see the first powdery mildew colonies since the longer you wait, the harder it is to get the disease under control. Make sure to apply sufficient spray volume to get good coverage since these are contact materials. Also, be careful during periods of hot weather as oils tend to block the stomata, preventing evaporative cooling of leaves so they can get very hot and may burn. During hot weather, use the 1 percent v/v rate. Other eradicant products with moderate efficacy are Kaligreen, MilStop, and Armicarb (potassium bicarbonate salts) and Oxidate (hydrogen peroxide). You can follow these with preventive fungicide applications to protect against new infections.

For visible downy mildew, the best options are Ridomil Gold MZ or Ridomil Gold Cu since these have excellent systemic and curative activity. However, keep in mind that the pre-harvest intervals (PHI) are long and may preclude their use at this time: 66 days for the MZ formulation and 42 days for the copper formulation. Phosphites such as Phostrol and ProPhyt are good alternatives – they have a zero-day PHI. Two sprays five days apart boost the curative and anti-sporulant potential. Be careful during hot weather or when plants are stressed as phosphites are salts and phytotoxicity is a possibility. Cut back on the concentration or wait until the temperature drops. Some other products like copper, Ziram, oils and salts can kill downy mildew sporangia on contact, but remember that sporangia are produced on the undersides of leaves and it may be more difficult to achieve effective coverage for that reason.

Things to remember

  • Apply treatments as soon as possible after symptoms are seen; regular and careful scouting is a prerequisite. However, waiting a little longer to ensure good spray conditions is a better option than spraying immediately under poor spray conditions.
  • If disease symptoms are showing up on leaves and shoots, you can assume that there is plenty of disease pressure to infect the fruit as well.
  • Avoid spraying systemic fungicides on heavily sporulating lesions since this is not very effective anyway and can encourage fungicide resistance development. Rather, apply a contact fungicide to kill the spores first and then follow up with systemic fungicide applications.
  • Remove infected clusters if possible and pull leaves around clusters to ensure good spray coverage of fruit zone and reduce humidity around clusters. This is especially important for Botrytis bunch rot and sour rot – be careful not too pull too many leaves which will leave the clusters vulnerable to sun-scalding.
  • Ensure thorough coverage of leaves and bunches, particularly for contact fungicides, which means increase spray volume, reduce air flow, reduce tractor speed, spray every row and adjust nozzles accordingly.
  • About four to five weeks after bloom, berries become mostly resistant to powdery mildew, downy mildew and black rot, although already-infected berries may show symptoms. There usually is no point in spraying clusters to protect them from new infections; however, some late infections may occur, especially on “straggler” clusters that are behind in their development. Also, some wine grape cultivars may remain susceptible to black rot up to eight weeks after bloom.
  • As berries ripen, they become more susceptible to Botrytis bunch rot and sour rot. Choose effective fungicides for control of these diseases. These are best applied before bunch closing, at veraison and two weeks before harvest. Clusters remain susceptible to Phomopsis throughout their development, but usually spores taper off and infection risk is low to nil after bunch closure. As infected berries ripen, however, they can rot and produce spores, leading to increased secondary infection risk close to harvest.
  • Apply fungicides at the highest labeled rate to ensure good post-infection activity.
  • Ensure forward protection of healthy plant parts by tank-mixing or following up with materials that have good protective activity.
  • Always read the label for the PHI, incompatibility with other products, and other restrictions.
  • Scout again to see if your treatment was effective, keeping in mind that newly developing infections may continue to manifest themselves for a week or more after the spray.

Dr. Schilder’s work is funded in part by MSU’s AgBioResearch.

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