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Pathogen Biology
The fungi that cause Leucostoma cankers on stone fruits, Leucostoma cinctum and L. persoonii, are ascomycetes in the order Diaporthales, family Valsaceae. The imperfect (anamorphic) stages of these fungi, Cytospora cincta (=Leucocytospora cinctum) and C. leucostoma (=L. persoonii), respectively, are commonly encountered in the field. The morphological differences between the two species are often difficult to discern. Pycnidia form in cankers and on killed twigs and branches distal to cankers, appearing as quickly as 2 to 3 weeks or up to 6 months after the death of the bark. In irrigated regions, late summer water stress of infected trees can accelerate pycnidial production. Pycnidia are black on the surface, gray to grayish-brown internally, and underlain by a black layer. Development of pycnidia is first evident as pinhead-sized swellings, the top of which ruptures to expose the disc of the stroma. The disc of L. persoonii has a white or frosted appearance (Figure 7), whereas that of L. cinctum is gray to brownish-gray. Mature pycnidia under moist conditions extrude flesh- to orange-colored tendrils, or cirri (Figures 7, 8), containing thousands of conidia (Figure 9). Exudation ensues within minutes after wetting. The sexual reproductive structures (perithecia containing ascospores) (Figure 10) form much later, often 2 to 3 years after the original formation of pycnidia.
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by The American Phytopathological Society