|MOEIN, S - Stellenbosch University
|MCLEOD, A - Stellenbosch University
Submitted to: Meeting Abstract
Publication Type: Proceedings
Publication Acceptance Date: 11/30/2014
Publication Date: 1/18/2015
Citation: Moein, S., Mazzola, M., Mcleod, A. 2015. Identification of external inoculum sources of apple replant pathogens. 49th Congress of the Southern Africa Society of Plant Pathology. p. 75.
Technical Abstract: Apple replant disease (ARD) is an important disease world-wide and occurs when old apple orchards are replanted with apple. The disease is mainly caused by biological agents, since fumigation alleviates symptom development. The main ARD causative agents are fungi (Rhizoctonia solani AG-5 and AG-6, a few binucleate Rhizoctonia spp. and several Cylindrocarpon spp.), parasitic nematodes (Pratylenchus) and oomycetes (certain Pythium spp. and Phytophthora), which have been reported worldwide. Although old orchard soils are the main source of ARD pathogens, contaminated nursery trees and irrigation water could be additional inoculum sources. This could explain why in some instances growers do not obtain good tree responses in fumigated orchard soils as well as the rapid pathogen re-colonization of fumigated soil within one year after planting. Apple nurseries and irrigation water were thus investigated as external ARD inoculum sources in the Western Cape region in South Africa. Roots of nursery grown stock were sampled from five sites and represented four different blocks per nursery with multiple rootstocks surveyed. Samples were collected during the spring (late September to mid-December) of 2013. Water was monitored at two points on a farm that included (i) the irrigation pipe within the orchard and (ii) the dam or river from which the irrigation pipe received its water. Isolation studies from nursery tree roots for oomycetes revealed the presence of several Pythium species including highly virulent P.irregulare and P. ultimum, moderately virulent P. dissotocum and a few non-pathogenic species. The percentage of trees infested with pathogenic Pythium species ranged from 35% to 70% across the different nurseries. Pythium irregulare was recovered from all nurseries and most nursery blocks (85%). Pratylenchus spp. were isolated from trees in all of the nurseries, however, the percentage of trees possessing root densities of 100 Pratylenchus/5g or greater varied from 0 % to 40% in the nurseries. There was no apparent correlation between rootstock cultivar and ARD pathogen infestation levels. In future, nursery roots will also be analyzed for the presence of R. solani AG-5, selected pathogenic Pythium spp. and the genera Cylindrocarpon and Phytophthora using quantitative real-time PCR (qPCR). Isolation studies from baited leaf discs and fruits in water samplings showed that Pythium species were present in all of the dams, and in 57% of orchard water samples. Phytophthora spp., which did not include the known apple pathogen Ph. cactorum, were only found in 18% of dams, but not in any orchard water samples. In total, 17 different oomycete spp. (14 Pythium and 3 Phytophthora) were identified in dam water, and six Pythium spp. in orchard water samples. Among these species, known ARD pathogens were not identified in orchard water samples, but moderately virulent P. dissotocum and weakly virulent P. litorale were identified infrequently in dam samples.