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Wilsonomyces carpophilus (Shot Hole)

Shot hole is caused by the fungal pathogen Wilsonmyces carpophilus.

Most severe on apricots, but occurrs on all stone fruit. The fungal patogen infects the leaves, twigs and fruits.

Symptoms

Infected leaves show small brown spots with reddish margins (about 1 mm diameter), these spots expand to larger circular lesions (about 3mm diameter).  These spots dry and fall out of the leaf, giving a shothole appearance. The infected twigs show clear- cut brown margins with a negrotic center, which does not drop out, but ooze large amounts of gum. Further on lignification of infected twings is hindered and the lesions will grow into cankers. In severe cases premature defoliation of the tree may result.

Fruits show firstly small circular, deep purple spots. As the disease progresses, the symptoms differ according to fruit type. On apricots the spots become brown, raised and rough, giving the fruit a scabby surface. On peaches and nectarines the scabs develop into deep indentations.
Infected fruit have spots of gum and in severe cases cracks in the skin.

The shothole fungus survives in infected buds. It is able to infect leaves, stems and fruits during cold, rainy weather periodes in spring and autumn. Rain periodes to infect healthy plant organs are needed.

The fungus is able to persist several years in the cankers or buds of infected twigs. Whenever conditions are favourable it may continue to grow, even during winter time. In spring time the conidia are splashed by rain to flowers and yound leaves and infect them. In unfavourable periodes (dry conditions) the conidia are still viable for several months. Rain is necessary for dispersal and humid conditions are needed for germination. The fungus is able to grow above 2°C.

When moisture is continuous for at least 24 hours and temperatures are above 2°C, conditions for infection are given. When temperatures are higher during the growing season, shorter periods of moisture are required for germination of the fungal pathogen; for example only 6 hours are needed at 25°C. Spores spread primarily by splashing water and can remain viable several months under dry conditions.  Under favorable conditions, spores can be produced from infected buds and stem lesions throughout the growing season. Most cultivars of peach, nectarine, apricot, and almond appear to be very susceptible. Cherry and plum are less susceptible and show only leaf and fruit symptoms when extended periods of moisture are present in late spring and early summer.

The FieldClimate.Com Infection model for shot hole disease shows the infection progress lines for weak, moderate and severe infections. The model is similar to the modelling of apple scab.  At the beginning of May a shot hole infection has been initialised by rain. The leaf wetness and the high relative humidity was lasting long enough to finish a weak and severe infection.