The summer of 2014 was exceptionally hot and sunny in the state of Washington. Given these conditions, Washington State University researchers say it is no wonder that reports of Concord blackleaf have been rolling in. In some cases, blackleaf has not been limited to juice grapes; there have also been reports of it on some wine grapes in certain locations.
In order to understand how to potentially manage or prevent blackleaf, WSU researchers say growers have to understand what it is and what it is not. It is not a nutritional deficiency. All of those old recommendations for applying potassium fertilizer to alleviate symptoms? Throw them out. It is not a result of mite feeding. Mites will cause a more browning and bronzing of leaves; and they can be controlled with a miticide. Spraying a miticide will not alleviate blackleaf. It is not a disease; not powdery mildew, not grapevine leafroll.
Blackleaf is a physiological disorder that results in the degradation of chloroplasts (cellular structures that conduct photosynthesis) and death of epidermal cells (the outer “skin” of a grape leaf). This degradation and death is caused by exposure to excessive short wave ultraviolet B (UVB) sunlight. Specifically, in blackleaf, the damage is a result of exposure to UVB when the leaves are not fully mature and therefore lack the waxy cuticle and build-up of sunscreen compounds that would naturally protect the tissue from damage.
In Washington, most of the damage that is seen in Concord vineyards as “black leaves” in September was actually from damage that was initiated in late June and early July. This is why blackleaf tends to be a bigger problem during years with reduced cloud coverage during those months. The damage takes time to fully manifest itself and display the “black leaf” symptoms. It is akin to the delayed muscle pain felt when starting a new sport or exercise regime.
Why do some blocks of Concord grapevines display severe symptoms of blackleaf when adjacent blocks have very little? Drought stress has been implicated in exacerbating blackleaf in Concord. Drought stress reduces vine transpiration, which in turn, can result in superheating of leaf tissue exposed to the sun. This high heat exposure can further damage chloroplasts and cells, accelerating the damage to the leaf tissue. As such, vineyards that experienced water stress do risk having more severe blackleaf symptoms. Water stress only enhances symptom development in vineyards that have already suffered from blackleaf damage, it does not cause it.
There are no current commercially acceptable techniques for preventing blackleaf in Concord vineyards. However, in years with few cloudy days and warm temperatures, the severity of blackleaf may be reduced with appropriate management of vine water stress.
For more information on blackleaf and potential ways to reduce symptoms, please see WSU Extension Publication EB0745, Blackleaf in Grapes (Olmstead et al. 2005).
Source: Washington State University