Pathogens, pathogenicity and epidemic conditions of cherry stem rot inYantai

Abstract:【Objective】Cherry stem rot, whose pathogen is Phytophthora nicotianae, represents a seriousdisease in the early stage of cherry seedlings. In these studies, we isolated and identified the pathogensfound in the field to investigate the disease resistance of different cherry varieties and the conditions ofthe disease epidemic. These researches can provide scientific strategies for disease control and breeding.【Methods】The pathogen P1639 isolated from the cherry stem was collected from the field in the Fushandistrict of Yantai. The phenotypic characteristics (colony, mycelium, sporangium and chlamydospore mor⁃phology) of the pathogen were monitored by observation and using a microscope. We observed the mor⁃phology and color of the strain P1639 after being cultured on a PDA or 10% V8 medium plate for 3 d. Themorphology and size of the mycelium on the slide glass were observed and measured with a microscope.Morphology and sizes of chlamydospores and sporangium were observed and measured with the micro⁃scope after being induced and produced. The genomic DNA of the pathogen strain P1639 was extractedand then sequenced with ITS primers. The strain P1639 was inoculated on cherry leaves according toKoch’s principle. A total of 17 different species (5 species and 1 intergeneric cross) of cherries were inoc⁃ulated with P1639, and the pathogenicity was determined. P1639 was cultured on a solid plate of 10% V8for 4 d and then the new cherry stems were inoculated using plugs of 6 mm diameter from the plate aftersurface sterilization. The stems were made moist with absorbent cotton after inoculation, and cultured inan incubator with 25 ℃ 12 h light conditions. The results were statistically analyzed after 3 to 7 d. Thepathogenicity of the strain P1639 to the leaves, the new stems and the woody stems of the cherry variety‘Van’were further determined by laboratory tests. The leaves, new stems, and woody stems of the cherryvariety‘Van’after surface sterilization were inoculated with plugs of 6 mm diameter from the plate. Theleaves were moistened in Petri dishes with wet filter paper after inoculation. The stems were purified withabsorbent cotton after inoculation. They were all cultured in the incubator with 25 ℃ 12 h light conditions.The results were statistically analyzed after 3 to 7 d. The occurrence of cherry stem rot in the cherry seedling field in Fushan district of Yantai was investigated for four years combining with local meteorologicaldata (temperature, humidity and rainfall) and then we analyzed the epidemic dynamics and epidemic condi⁃tions of the disease.【Results】The colony of the pathogen Phytophthora nicotianae strain P1639 causingcherry stem rot disease showed a white color and irregular edges and the aerial hyphae was exuberant show⁃ing a slow growth on the PDA plate. The colony displayed a white color and regular edges and the aerial hy⁃phae were exuberant showing a much faster growth on the 10% V8 plate. Under the microscope, the hy⁃phae were colorless and transparent, with a diameter of 2 to 6 μm; the sporangia (37.5-62.5 μm × 30-50μm) were round or oval with obvious papillae; most of the chlamydospores (19-42 μm, average 30.4 μm)in the top or middle mycelium were round and a few were oval. The morphological and molecular identification showed that the strain P1639 was the cherry stem rot pathogen Phytophthora nicotianae. The pathoge⁃nicity of the isolated strain P1639 to 17 varieties of cherries was tested in the laboratory. The resultsshowed that P1639 expressed different levels of pathogenicity to‘Wanfeng’‘Mingzhu’‘Lapins’‘Tieton’‘Brooks’‘Summit’‘Santina’‘Heizhenzhu’‘Hongdeng’‘Sunburst’and‘Van’.‘Wanfeng’‘Mingzhu’and‘Lapins’showed the strongest susceptibility with lesion length of 3.21 and 4.22 cm;‘Tieton’and‘Brooks’showed the second susceptibility with lesion length of 2.23 cm and 1.96 cm separately.‘Summit’‘Santina’‘Heizhenzhu’‘Hongdeng’‘Sunburst’and‘Van’showed lighter susceptibility with lesionlengths from 0.91 to 1.12 cm. The strains which showed no pathogenicity were‘Daqingye’‘Mahaleb’‘Gisela 6’‘ZY-1’‘Dongbei flowering cherry’and‘Yingzhenwang’. The pathogenicity of the isolatedstrain P1639 to the leaves, new stems and woody stems of the cheery variety‘Van’was further determinedin the laboratory. The results showed that P1639 could successfully infect the leaves and new stems, butcould not infect the woody stems. According to the investigative results of 4 years, the cherry stem rot disease began in the middle of July, and ended in the middle of August, which had the highest average temper⁃ature (above 25 ℃) during the year. During the period of the middle to the end of July, if there was largerainfall and high humidity, the cherry stem rot disease would occur in a large outbreak, otherwise, the disease almost did not occur.【Conclusion】All these results indicate that the pathogen showed a strong pathogenicity to Prunus avium but no pathogenicity to P. pseudocerasus, P. Mahaleb, P. Serrulata, P. cerasus and‘ZY-1’, an intergeneric cross between P. cerasus and P. canescens. The pathogen could successfully infectthe leaves and new stems but not the woody stems of P. avium. The epidemic conditions of cherry stem rotwere closely related to temperature, rainfall and relative humidity. Meanwhile, great outbreaks could appear in the rainy summer with the presence of high temperatures.