No. 11 — June 3, 2019


Post Bloom Disease Management

close-up of a Vidal blanc leaf showing lesions
Photo: Phomopsis viticola lesions on Vidal blanc on June 6, 2019. Photo credit: D.S. Volenberg

The grape berry crop is vulnerable to a number of disease-causing pathogens until a period of time passes after bloom. Currently, most grape cultivars are approximately 10 days post bloom, and grape berries are buckshot to pea size. The grape berries are susceptible to black rot, downy mildew, powdery mildew and anthracnose. There is reduced risk from Phomopsis infections at this time since the spore load declines around the period of fruit set. Post bloom disease management should continue until the grape berries have developed age related resistance to downy mildew, powdery mildew and black rot. The berries develop resistance to these pathogens approximately 4 to 6 weeks after bloom. Although the berries develop resistance, the leaves, shoots, tendrils and other green tissues remain susceptible to infection throughout the growing season. The period from immediate pre-bloom through post-bloom is when you should use the most effective fungicides to protect the crop.

There are number of fungicides that will provide protection during the post-bloom period. Consider the strobilurins (Abound, Flint, Sovran) or the Sterol inhibitors (Rally, TebuStar) or combination fungicides (Inspire Super, Pristine, Quadris Top, Revus Top). Take the time to familiarize yourself with the effectiveness of these fungicides on each pathogen (See page 91-92 in the 2019-2020 Midwest Fruit Pest Management Guide).

The weather forecast through June 14 likely will provide ideal environmental conditions for powdery mildew infections. The probability of precipitation will be low and both daytime and nighttime temperatures will be below normal for this time of year. These below normal temperatures will be ideal for powdery mildew infection and disease development. Ideal temperatures for powdery mildew infection are between 68 to 77°F. Powdery mildew disease infection and development often begins in shaded areas within the canopy. When scouting be sure to examine shaded leaves within the interior of the canopy.

With increased canopy growth and development coupled with fruit set means it is time to evaluate your spray coverage. This can be accomplished using some water sensitive cards stapled to leaves within the canopy. Also check your nozzles and nozzle orientation. Nozzles should be replaced when the flow rate exceeds the recommended flow rate by 10% or more.

In summary, the grape berry is at a critical developmental stage and needs to be protected from infections from pathogens. The two most critical protective fungicide sprays are immediate pre-bloom and the post bloom protective spray. These two protective sprays are when you should apply the most effective fungicides to control critical plant pathogens. As the canopies grow be sure that your sprayer is providing adequate spray coverage. If spray deposition is poor then consider re-calibrating your sprayer. Until the grape berries are 4 to 6 weeks post bloom and develop age related resistance, protective fungicides are imperative.

What Does All the Rain and Flooding Mean for the Grape Crop?

Vineyard with standing water on the ground between rows
Standing water in low areas in vineyards has been a common site in May and early June in Missouri. Photo credit: D.S. Volenberg, June 6, 2019.

The majority of Missouri has experienced substantial rainfall throughout April and May. For the most part vineyards have come through in good shape except for some erosion and wheel tracks from spraying when the soil is saturated. The main concern from excessive rainfall is when vineyards remain flooded for an extended period of time. Standing water may result from excessive rainfall, river flooding and vineyards established on poorly drained soils. If grapevines are flooded for extended periods then damage may include reduced productivity, vine collapse and vine death. Standing water should be removed from vineyards using pumps or other methods. When vines experience standing water the air spaces within the soil becomes filled with water and the soil becomes anaerobic. In this environment the grapevine roots cannot respire and the roots die.

The excessive rainfall will also negatively impact pre-emergent herbicides. Pre-emergent herbicides can be lost through leaching or washed away with surface water runoff. Saturated soil conditions also provide an ideal environment for the degradation of some herbicides compared to well-drained soils. This is true for dinitroanaline herbicides Surflan (oryzalin) and Treflan (trifluralin). As the soils dry out be prepared for a flush of weeds. Be prepared to control weeds with post-emergent herbicide applications.

Cool nighttime temperatures have resulted in excessive dew conditions. The early morning dew provides the right environment for disease infection. The major disease pathogens require wet tissue (free water) for infections to occur. The disease patho-gens of concern are downy mildew, black rot, phomopsis and anthracnose. Therefore in the absence of rainfall, dew still provides an environment for infection to occur.

Excessive rainfall has likely reduced the amount of fertilizers that were applied during the early part of the growing season. Nitrogen likely has been lost through leaching and sub-surface runoff. Some soil applied potassium and phosphorous likely has been lost through erosion and sub-surface runoff. Fortunately grape growers had the oppor-tunity to evaluate grapevine nutrient status during bloom. For those grape growers that missed petiole sampling at bloom, nutrient status of the grapevines will have to be done by visual observation. In this scenario growers should evaluate grapevine vigor and leaf color visually. If grapevines begin to show nutrient deficiency symptoms then a petiole sample can be taken. Two petiole samples are needed, the first sample will contain petioles from grapevines showing the deficiency and the second sample will contain petioles from grapevines that appear healthy. The petiole sample results will provide you the opportunity to compare nutrient levels between deficient and healthy grapevines.

Norton and Mancozeb Phytotoxicity

In September of 2018, Lucie Morton and I, as well as other colleagues, had an email discussion about Norton and mancozeb phytotoxicity. Fact or fallacy? At the time, a grower in Virginia had reported that older Norton leaves would turn a distinctive tan or light brown on the underside of the leaves after being treated with mancozeb. These older basal leaves after about 30 days would get necrotic edges, curl and fall from the shoot.

From this initial conversation, we also learned that mancozeb is a profungicide. When mancozeb comes in contact with water ethylene bisisothiocynate sulfide (EBIS) is released. Further when EBIS is exposed to UV light ethylene bisisothiocynate (EBI) is formed. If these compounds find there way into the plant tissue then phytotoxicity is likely to occur.

This week, a grower in Missouri reported similar phytotoxicity symptoms on Norton after applying mancozeb.

I would like to know if other Norton growers have observed any phytotoxicity on Norton after applying mancozeb. Please contact me if you have observed phytotoxicity after applying mancozeb and please document the “phyto” with pictures.

D. S. Volenberg

Cumulative Growing Degree Days for the Seven Grape Growing Regions of Missouri from April 1 to June 3, 2019

Region Location by County Growing Degree Days1
2019 2018 30-year Average
Augusta St. Charles 821 919 810
Hermann Gasconade 812 879 776
Ozark Highland Phelps 881 954 825
Ozark Mountain Lawrence 877 956 821
Southeast Ste. Genevieve 863 924 841
Boone 792 924 767
Western Ray 705 881 741

1 Growing degree days at base 50 from April 1 to June 3, 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.