No. 11 — June 3, 2019
What Does All the Rain and Flooding Mean for the Grape Crop?
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.