No. 6 — April 29, 2019


Rainfall and Fungicide Efficacy

The amount of rainfall that occurs reduces the amount of fungicide residue remaining of the grape leaf surface. Simulated rainfall events of 0.1, 0.5, 1, and 2 inches removed 25, 30, 65, and 75 percent of one-day old Ziram fungicide residues (Annemiek Schilder, 2010). Even though Ziram residues were removed by rainfall, Phomopsis control remained above 90 percent for all rainfall events. Similarly, this was demonstrated for Captec and Abound. From this research some guidelines were established for when a fungicide should be reapplied in regards to rainfall amount.

  • If 2 inches or more of rainfall on a recently applied protectant fungicide, then the fungicide should be reapplied.
  • If the protectant fungicide is greater than or equal to 7 days old and 1 inch of rainfall, then the protectant fungicide should be reapplied.
  • All fungicides, which includes both protectant and systemic fungicides, must dry completely on the grapevine prior to a rainfall event.

Keep an Eye on the Weeds

Excessive rainfall amounts can reduce the effectiveness of pre-emergent herbicides. Most all pre-emergent herbicides are activated by 0.25 to 0.5-inches of rainfall. Then subsequent rainfall amounts in the range of 1 to 2 inches help distribute the pre-emergent herbicide as a “protective barrier” in the first couple of inches of the soil. When rainfall amounts are excessive the pre-emergent herbicide becomes less effective. Depending of the chemistry of the pre-emergent herbicide, the herbicide can be leached deeper into the soil profile thereby mov-ing out of the zone of germinating weed seeds. Some pre-emergent herbicides bind tightly to the soil and are washed away during excessive rainfall via erosion.

Keep an eye on annual weed growth and be ready to apply a post-emergent herbicide if needed. Use caution if considering another pre-emergent herbicide application as many pre-emergent herbicides have extended periods between reapplication. For example, Chateau SW has 30-days between applications, Zeus Prime has 60 days between applications.

With the current saturated soil conditions weed growth and vineyard floor grass should be allowed to grow to help remove excess soil moisture. As the soil dries down, be prepared for a flush of annual weeds.

Focus on Phomopsis, Black Rot and Downy Mildew

With wet, cool, and cloudy conditions prevailing through next week, Phomopsis viticola is a concern. Additionally, black rot and downy mildew are the other two diseases that a threat. All three of these pathogens need the grape tissue to be moist in order for infection to occur. Phomopsis needs 6 hours of wetness with air temperatures between 60 to 68 degrees for infections to occur. The fungus overwinters on infected canes or rachises.

Photo: Leaf, top down, to show phomopsis lesions
Phomopsis lesions on Vignole leaf from the New Haven, Mo., on May 3, 2019.
Photo credit: D.S. Volenberg

Black rot overwinters on mummified fruit or as cane lesions. Rainfall events triggers the release of spores from the mummy berries or cane lesions. The spores can travel to susceptible grape tissue by splashing raindrops and wind. Black rot spores can infect all green tissue, infection is dependent on the air temperature and the duration of wetness of the tissue. Only 9 hours of tissue wetness are needed when air temperatures are between 60 to 70 degrees for infection to occur.

Similar to Black rot, Downy mildew Plasmopara viticola zoospores are released by rainfall and can infect susceptible grape tissue when moisture is present. Disease development is dependent on temperature with the optimum temperature being between 64 to 76 degrees F. Downy mildew overwinters on infected leaf tissue.

In summary, all three of these pathogens have similar environmental requirements for infections to occur. Rainfall is needed to dislodge spores, grape tissue needs to remain wet for a period of time and air temperatures range from 60 to 76 degrees.

Sanitation should be part of your management program to control these three diseases. Canes infected with Phomopsis should be pruned from vines and destroyed during the dormant pruning period. To manage Black rot, all rachises and mummy berries should be removed from grapevines during the dormant pruning period. Protectant fungicides such as mancozeb should be applied starting at 1/2" to 1” shoots, especially if wet periods are forecast.

Tips to Minimize Soil Compaction During Spring Spraying Events

Continued rainfall events have resulted in saturated soil conditions across most of Missouri vineyards. Many vineyards may be nearing the end of the first or second cover spray of fungicide an another cover spray will need to be applied. Taking heavy equipment such as tractors and airblast sprayers onto saturated soils causes the collapse of soil aggregates. In turn surface soil becomes compacted, rutting occurs and the subsoil becomes compacted.

Soil compaction reduces water infiltration, root development and is known to reduce yield in row crop grains such as corn and soybean.

To minimize soil compaction during spraying

  • Reduce the amount of spray solution in the sprayer tank. This will reduce the amount weight on the axles but increase the amount times you must mix fungicides. If you have a smaller tractor that has the horsepower capacity to operate your sprayer consider using a smaller lightweight tractor.
  • Adjust tire air pressure on your sprayer to match the axle load. Larger tires with lower air pressure provide more surface area and reduce pressure on the soil surface.
  • Depending on your soil conditions and vineyard site, you may need to consider applying fungicides using an alternate row cover. Meaning you spray every other row. If considering alternate row cover, make sure to adjust your sprayer nozzles, baffles, fan speed to provide coverage to the distant vineyard rows. Be aware that there are risks involved in alternate row cover such as poor coverage. I do not recommend alternate row cover sprays precisely because of poor coverage. However with ponding water in some vineyards this may be your only choice in some areas where water is ponding.

Cumulative Growing Degree Days for the Seven Grape Growing Regions of Missouri from April 1 to April 29, 2019

Region Location by County Growing Degree Days1
2019 2018 30-year Average
Augusta St. Charles 262 164 259
Hermann Gasconade 265 167 258
Ozark Highland Phelps 298 177 276
Ozark Mountain Lawrence 296 186 270
Southeast Ste. Genevieve 279 171 279
Boone 261 155 243
Western Ray 249 141 231

1 Growing degree days at base 50 from April 1 to April 29, 2019. Data compiled from Useful and Useable at Click on link below to determine growing degree days in your area.

To determine the number of growing degree days accumulated in your area since April 1, use this tool.