Phoma leaf spot of grape

Phoma diplodiella is a fungal pathogen responsible for a disease commonly known in viticulture as Phoma leaf spot or dry rot of grapevine. It is an ascomycete fungus that primarily affects the woody parts of the vine, including canes, trunks, and occasionally the clusters. In severe cases, the disease can lead to the death of entire arms of the vine, causing significant economic losses.

Symptoms of the infection typically appear as small, dark spots or lesions on the surface of the canes. As the infection progresses, these spots expand into greyish-brown patches, often accompanied by the development of numerous tiny black structures known as pycnidia. These structures are the hallmark of the fungus and serve as the source of primary inoculum for subsequent infections within the vineyard.

The biological cycle of the pathogen is centered on its ability to overwinter within infected plant tissues or necrotic wood debris. When spring temperatures rise and moisture levels increase, the pycnidia release large numbers of spores. These spores are disseminated via wind or splashing rain to new, healthy green shoots, where they penetrate the vine through natural openings, stomata, or wounds caused by pruning or winter frost.

Environmental conditions play a critical role in the prevalence and severity of Phoma infections. High humidity, frequent rainfall, and stagnant air within the canopy are primary drivers for spore germination and infection establishment. Vineyards that are poorly pruned or lack adequate spacing are disproportionately susceptible to rapid spread, especially during cool, wet spring weather conditions.

Control of Phoma diplodiella requires an integrated approach focusing on both sanitation and chemical intervention. Growers should remove and destroy infected prunings to reduce the amount of overwintering inoculum in the vineyard. Cultural practices that improve air circulation, such as proper canopy management and timely leaf thinning, are essential. Preventive fungicide applications, particularly those containing copper or effective systemic agents, should be timed during the early growth stages to protect young tissue.