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Several Trichoderma species can act as biocontrol agents and hold the potential to control soilborne diseases through different modes of action. Little is known about the colonization pattern of Trichoderma atroviride in grapevine roots and activation of induced systemic resistance in planta. A laboratory model was developed to assess root colonization and its impact on grapevine defence activation. Rootstock cuttings from 1-year-old dormant canes were inoculated with conidium suspensions of T. atroviride T-77 or T. atroviride USPP T1, and host and inoculum colonisation were assessed after 21 d. The two strains of T. atroviride were re-isolated from the treated plants (from 70% of the roots and 20% of crowns). Colonization rates did not depend on the Trichoderma strain or rootstock cultivar. However, up-regulation of targeted defence genes was dependent on the inoculated Trichoderma strain and rootstock cultivar. Furthermore, in leaves of rootstock cultivars ‘US 8-7’ and ‘Paulsen 1103’, genes were up-regulated which encode for PR proteins involved in plant defence or production of stilbenic phytoalexins. Trichoderma atroviride T-77 was transformed with tdTomato fluorescent protein to allow visualization by confocal laser scanning microscopy. These results give new insights into the mechanisms of grapevine-Trichoderma interactions, and allow detection of establishment of potential biocontrol agents within host tissues.

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.

Nineteen species of Phaeoacremonium have been associated with grapevines in South Africa, of which only six species have been confirmed as pathogens through pathogenicity tests conducted on field-grown grapevines. This study determined the pathogenic status of ten Phaeoacremonium spp. recently found for the first time on South African grapevines. These were: Pm. australiense, Pm. austroafricanum, Pm. fraxinopennsylvanicum, Pm. griseo-olivaceum, Pm. griseorubrum, Pm. iranianum, Pm. italicum, Pm. prunicolum, Pm. scolyti and Pm. sicilianum. In the pathogenicity tests, Ph. parasiticum was used as the positive control, and sterile water as the negative control. Up to three isolates were used per species, depending on isolate availability. Freshly cut pruning wounds in a 9-year-old Cabernet Sauvignon vineyard in Stellenbosch, South Africa, were inoculated with 200conidia of each fungus per wound. Inoculated pruning wounds were removed after 18 months, cut longitudinally and lesion lengths were measured. Re-isolation proportions were determined by conducting isolations from inoculated spurs. All the inoculated isolates successfully colonized pruning wounds, and caused lesions that were significantly different from the negative control. All isolates were re-isolated at proportions varying from 28.6 to 85.7%. Phaeoacremonium griseo-olivaceum STE-U 7859 produced the longest lesions (mean = 79.5 mm) and Pm. iranianum STE-U 6998 the shortest (62.0 mm). No statistically significant differences in mean lesion lengths were observed between the inoculated species. There were also no significant differences between isolates of the same species, except in Pm. prunicolum where isolate STE-U 5968 produced longer lesions (mean = 77.3 mm) than STE-U 7857 (62.3 mm). This study confirmed the capabilities of all the tested Phaeoacremonium spp. to infect grapevine pruning wounds and cause lesions. The study also confirmed the importance of pruning wounds as ports of entry by these pathogens into host plants.

Black foot disease of grapevine is a relatively new, and as yet poorly known disease affecting vines in various countries where grapevines are cultivated. The causal organisms, their distribution, associated symptoms, known epidemiology and possible management strategies are discussed. Specific attention is also given to the taxonomy of the fungi involved, and the detection methods being developed to facilitate rapid identification of these pathogens.