No. 13 — June 17, 2019
Rupestris speckle is a physiological disorder that affects certain grape cultivars. Grape cultivars that are affected have Vitis rupestris in their heritage. Many of the hybrid grape cultivars have some Vitis rupestris in their genetic background. Grape cultivars affected by the disorder show different degrees of speckling (Figure 1). Valvin muscat typically displays moderate to severe speckling. Whereas, Chambourcin displays minor to moderate speckling. The physiological cause of the disorder is not known, but the disorder has been linked to stress that causes reduced or low vigor. The degree of the stress likely plays a role since Rupestris speckle does not appear every year and the degree of speckling often is different from year to year. The last time the disorder was reported by multiple growers in Missouri was in 2016.
What makes Rupestris speckling confusing is the degree of speckling that often resembles many fungal diseases. The speckles resemble black rot (Guignardia bidwellii) lesions. You can distinguish black rot from Rupestris speckles by looking at the lesions with a 10X handlens. Black rot lesions will contain black dots or fruiting bodies (pycnidia) whereas Rupestris speckles will be void of lesions. Other disease symptoms that resemble Rupestris speckles are Septoria leaf spot, Septoria ampelina and bacterial leaf spot, Xanthomonas campestris pv. viticola. Septoria leaf spot can be distinguished from Rupestris speckles once again by using a 10x hand lense to observe if fruiting bodies are apparent within the lesion. In addition, Septoria leaf spot mainly affects American Vitis species as well muscadine grapes. Bacterial leaf spot affects young growing shoots, leaves and berries. Whereas, Rupestris speckles mainly affects older leaves.
Since Rupestris speckle is a physiological disorder there are no control recommendations.