Aerial inoculum patterns of Petri disease pathogens in South African vineyards and rootstock mother blocks

Abstract

Petri disease is caused by the xylem inhabiting fungi Phaeomoniella (Pa.) chlamydospora and several Phaeoacremonium (Pm.) species. Pruning wounds are known host ports of entry for aerial spores of these pathogens. However, knowledge is lacking on occurrence of these pathogens as aerial inoculum within South African vineyards. This study determined when spores of Petri disease pathogens are released in Western Cape Province vineyards and how these spore release events coincided with pruning activities when infections could occur. The research was conducted for two seasons from mid-May to early December 2012 and from mid-March to early December 2013. Microscope slide spore traps were affixed to arms of infected vines in six vineyards and mother vines in two rootstock mother vine nurseries. The slides were replaced weekly and fungal spores were retrieved from them, cultured, counted and identified. Colonies resembling those of Pa. chlamydospora and Phaeoacremonium spp. were subcultured for further molecular identification. Species of Phaeoacremonium were identified by amplification of the partial beta-tubulin gene. Taqman probes and primers were developed to facilitate fast detection of the most frequently occurring species (Pm. minimum, Pm. parasiticum and Pm. sicilianum), using real-time PCR. Petri disease pathogens occurred throughout the periods investigated. Phaeomoniella chlamydospora and Pm. minimum were trapped in all vineyards. A total of 14 Phaeoacremonium species were identified, with the greatest diversity ever recorded in vineyards, including Pm. australiense, Pm. griseo-olivaceum, Pm. griseorubrum, Pm. inflatipes, Pm. iranianum, Pm. italicum, Pm. minimum, Pm. parasiticum, Pm. prunicola, Pm. scolyti, Pm. sicilianum, Pm. subulatum, Pm. venezuelense and Pm. viticola. Of these, only Pm. minimum and Pm. inflatipes have been reported as aerial inoculum within vineyards. Spore release coincided with winter and spring pruning activities. The occurrence of six Phaeoacremonium species in rootstock mother vine nurseries highlights the high risk of pathogen spread through infected nursery material. This is the greatest Phaeoacremonium species diversity ever recorded in vineyards and the first detection of Phaeoacremonium species aerial inoculum in grapevine rootstock mother vine nurseries. The high species diversity and frequency of spore release in vineyards and rootstock mother vine nurseries coinciding with traditional pruning practices emphasizes the need to develop effective wound protection strategies to avoid infection of unprotected grapevine pruning wounds.