Venturia inaequalis

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Venturia inaequalis asexual spores on crabapple leaf (SEM, 1500x) (click on image to enlarge it)
Author(s): Charles Krause, USDA-ARS
Source: Wikimedia Commons

Venturia inaequalis (Cooke 1866) G. Winter 1897

The fungus causes apple scab, a very important fungus disease which has a world-wide distribution in apple growing regions. The infections result in necrotic, blister-like lesions on leaves and fruits. These have a distinct margin and on fruits might contain cracks through which other pathogens can enter. Infected leaves drop prematurely and the quality of infected fruits is reduced. Moist and rainy conditions promote infections and epidemics. Total yield losses might reach 70% or more. Infections also reduce the storage life of apples.

Vernacular names
• Deutsch: Apfelschorf
• English: apple scab
loquat scab
• Español: sarna del manzano
roña del manzano
• Français: tavelure du pommier
• Português: sarna da macieira

Resistant cultivars and fungicides are used for control. In addition, removing fallen leaves through sucking devices in autumn is recommended. In spring, airborne ascospores infect the buds and leaves. After several weeks, the fungus produces conidia which can spread the infection, e.g. through rain splashes. Conidia production continues throughout the growing season and one lesion might release up to 10,000 spores.

The conidia are approximately 15-20 x 8-10 µm large. In autumn, black and spherical pseudothecia are formed, about 0.2 mm large, which persist during the winter on the fallen leaves. In spring, sexual reproduction involving two mating types takes place and ascospores are produced. Ascospores are brown and contain 2 cells of "unequal" size. They have a characteristic "footprint" shape and are around 10-15 µm long and 5-7 µm wide.

Fusicladium pomi
Fusicladium pyracanthae
Passalora pyracanthae
Spilocaea eriobotryae
Spilocaea pomi
Spilocaea pyracanthae