Histopathological study of the effect of post leaf fall application of urea on saprotrophic development of the cherry leaf spot pathogen, Blumeriella jaapii

Summary

The formation and development of over-wintering structures of the leaf spot pathogen Blumeriella jaapii in urea treated leaves of sour cherry (Prunus cerasus) during autumn, early winter and in May were studied histopathologically and compared to leaves treated with water. Resin embedding using glycol methacrylate (GMA) and subsequent staining of sections with Amido Black were used. Macroscopic and microscopic studies of B. jaapii in water treated leaves showed that the development of the over-wintering stages begins randomly in individual leaf spots both pre and early post leaf fall. Saprotrophic growth and stroma formation in leaves is completed between six to eight weeks after leaf fall. During the following spring stromata normally develop apothecia or winter acervuli which release ascospores and winter conidia, respectively. However, treatment of infected leaves with urea at about leaf fall in autumn leads to a reduction in the saprotrophic hyphal growth of B. jaapii, inhibiting its subsequent development into stromata, and thereby reducing primary inoculum production. The results indicate that control of primary infections of cherry leaf spot might be achieved with the use of urea applied to the foliage within a period of at least four weeks around leaf fall.