Cherry leaf spot

Abstract: Growth chamber and orchard experiments were carried out to clarify the response of the cherry leaf spot pathogen, Blumeriella jaapii, and microorganisms in the leaf litter to two levels of urea (2.5% or 5%) applied post leaf fall to sour cherry leaves. In general, urea application reduced the development of B. jaapii measured as biomass by quantitative PTA-ELISA in the leaf litter and spore counting in the spring and increased the overall microbial biomass (measured with biomarker phospholipid fatty acids (PLFAs)) and fungal activity (assessed as β-N-acetylglucosaminidase activity). The biomass increase of all groups of microorganisms in the litter generally began immediately after application of urea and, with the notable excepts of B. jaapii and Gram-negative bacteria containing the biomarker PLFA cyclo17:0, continued after the ammonia and pH levels had stabilized to levels similar to the control leaves approximately 10 days later. Application of 2.5% urea increased the biomass of most groups of saprotrophic microorganisms and accelerated litter decomposition to a higher extent than application of 5% urea and during the first week after treatment applications the 5% urea level inhibited the total microbial biomass. This may be ascribed to ammonia toxicity as 5% urea resulted in a markedly higher ammonia elevation than 2.5% urea, the first week after application. From then onwards in both the 2.5% and 5% treatments the fungal and Gram-positive communities benefited from a lowered C:N ratio, increasing their activities approximately 2 times compared to a water-treated control. Inhibition of B. jaapii coincided with the period of urea breakdown in which elevated levels of ammonia and leaf pH were measured in the treated leaves. This period lasted for approximately 10 days after which the biomass of B. jaapii in the treated leaves continued to decrease at a slow but faster rate than in the untreated leaves. Our results indicate that the urea application caused an elevation in leaf pH and ammonia levels, which together with an acceleration in litter decomposition, adversely affected the saprotrophic growth of B. jaapii, leading to reduced production of ascospores and winter-conidia the following spring.

Abstract: A detached leaf disk assay was developed for screening sour cherry (Prunus cerasus) cultivars for resistance to cherry leaf spot (Blumeriella jaapii). This assay was used to characterize the events occurring in cv. Montmorency (susceptible) and cv. GiSelA 6 [GI 148-1] (resistant) host-pathogen interactions, and to develop a disease scoring scheme to categorize disease severity in sour cherry trees. Forty-three seedlings grown from seeds collected in Russia were screened for resistance using the scoring scheme. Cross infection studies were also carried out with leaf spot isolates from four other Prunus species. These studies were made possible by the development of a new method of culturing B. jaapii isolates, using cherry fruit agar. This method enabled large amounts of inoculum to be produced within 2 weeks rather than a couple of months as required previously. In ‘Montmorency’, disease symptoms were first observed 4 days postinoculation (dpi), with the appearance of small white spots on the und...