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grapegrowing Patty Skinkis, Oregon State UniversitY GRAPEVINE PROBLEMS Leaf spots not caused by insects or disease BY Fritz Westover, Texas A&M University; Jim Wolpert, University of California, Davis D amage or disorders of grapevines can arise from a wide array of causal factors. Sometimes a spot on a leaf or berry can be caused by an insect or a biological plant pathogen such as a fungus or bacteria, and other times the spot can be produced by human or environmental factors. A disorder on a grapevine that is not caused by a living pest or pathogen is referred to as an abiotic disorder. Diagnosing a disorder Before diagnosing a grape problem as abiotic in nature, any potential grape pests or diseases that may have produced the symptom must be ruled out. Several guides such as the "Compendium of Grape Diseases" by APS Press or "The Vineyard Doctor," the upcoming online grapevine problem diagnostic tool for eViticulture, are useful tools for diagnosing pest or disease symptoms. Some pests or diseases will produce symptoms on grapes that are commonly confused with abiotic disorders, so it is important to consult your local agricultural extension agency (or licensed pest control advisor) to confirm a diagnosis prior to implementing treatment. Spotting on these pre-véraison grape berries resulted from inadequate cleaning of the spray tank before mixing a new fungicide and applying the product. growth. Contact herbicides such as glufosinate or paraquat can produce brown necrotic spots on leaves, but generally do not result in vine death if the contact is limited. Note that not all herbicide damage may be the result of grower applications. Drift of systemic herbicides such as glyphosate can cause more severe damage, resulting in leaf distortion and possible vine death. Phenoxy herbicides such as 2,4-D can drift from agronomic crop fields in the vicinity of a vineyard, causing irreversible distortion to leaves and, in some cases, vine death. If your vineyard is irrigated, test the water quality periodically for impurities. High salt content in irrigation water or salt buildup in soil also can cause leaf scorch or vine death if not detected early. example, may cause oxidant stipple on the upper sides of leaves of susceptible grape varieties if atmospheric levels are elevated in the region surrounding a vineyard. Hydrogen fluoride also has caused leaf burn or necrosis in vineyards adjacent to industrial facilities where such atmospheric pollutants are produced. Drought is another environmental factor that can severely limit vine growth. Drought conditions can reduce nutrient uptake in vines, causing nutrient deficiencies and leaf discoloration. Long periods of drought can lead to death of vine shoot tips, stunted vine growth and delayed fruit maturation. Other environmental hazards include hail, sunburn, frost and freeze injury. Environmental disorders Nutritional imbalances Environmental factors also may be the cause of spots on leaves. Ozone, for Grape leaf discoloration or deformation can also be caused by nutritional deficiencies or Chemical disorders 68 p r acti c al w i ne ry & v i n e yard APRIL 20 13 Fritz Westover, Texas A&M University Leaf spots are one of the most commonly occurring symptoms of abiotic disorders. Leaf spots can be caused by unintentional drift or overspray of herbicides, or improper mixing of pesticides or fertilizers sprayed directly onto vines. Even when growers apply a product that was harmless in the past, fluctuations in weather or product rates can lead to unintended damage to vines. For example, many sulfur formulations can cause leaf scorch if applied during hot, humid conditions, whereas copper fungicides can cause similar injury if sprayed during cool, wet conditions. Also, failure to clean the spray tank before applying another product may lead to misapplication and leaf and cluster spotting. Herbicide drift is likely the most common cause of leaf spots or misshapen leaf Herbicide damage (clockwise from top left): Simizine, glyphosate and phenoxy (bottom two).