The role of arthropods in the dispersal of trunk disease pathogens associated with Petri disease and Esca

[摘要] ENGLISH ABSTRACT: Petri disease and esca are devastating grapevine trunk diseases and compromise thesustainability of viticulture world-wide. Despite being extensively studied, knowledge ofinoculum sources and mechanisms of spread of the causal pathogens is limited. Arthropodshave been suspected to play a role in the spread of Petri disease and esca pathogens.However, little information is known about the extent to which arthropods are associatedwith these pathogens. This study aimed to determine whether arthropods occurring within oron declining grapevines, are associated with trunk disease pathogens and to identifyarthropods associated with pruning wounds. The potential of selected arthropods to act asvectors of trunk disease pathogens was also investigated.Two vineyards exhibiting grapevine trunk disease infections were sampled weeklyfor two years for collection of arthropods. Arthropods were collected using pruning woundtraps, visual searches as well as trunk and cordon traps. Fungal spores from surfaces ofarthropods were collected in water. Samples were subjected to nested PCR using primersPm1/Pm2 and Pch1/Pch2 to verify the presence of Phaeoacremonium spp. andPhaeomoniella chlamydospora, respectively. Water samples were also cultured andgrapevine trunk disease pathogens obtained were identified by sequencing the internaltranscribed spacers 1 and 2 and the 5.8S rRNA gene or the partial beta-tubulin gene. A totalof 10 875 arthropod individuals, belonging to more than 31 families, were collected fromdeclining grapevines. The most abundant arthropods included millipedes, ants, spiders andbeetles. Portuguese millipedes and cocktail ants were associated with fresh grapevinepruning wounds. Thirty-three percent of the 5677 water samples analysed, containedpropagules of pathogens associated with Petri disease and esca. Of these, 37 % wererecovered from millipedes, 22 % from cocktail ants, 15 % from spiders and 10 % frombeetles. All the major groups of grapevine trunk diseases were detected on the arthropods.Phaeoacremonium species were detected in 1242 samples while Phaeomoniellachlamydospora was identified from 855 samples. Other fungi isolated included members ofthe Botryosphaeriaceae, Diatrypaceae and Diaporthales.The potential of grapevine sap as a food source for Portuguese millipedes andcocktail ants was investigated, in vitro. Millipede individuals were offered a choice betweenwater and grapevine sap while ants in nests were presented with grapevine sap, tuna andwater and monitored for ingestion of sap. Both taxa preferred grapevine sap over the other food items, indicating close association with pruning wounds. Subsequently, the ability ofboth taxa to transmit a DsRed-transformed Phaeomoniella chlamydospora isolate to freshpruning wounds of canes in polystyrene strips, floating in water, and potted vines wastested. Arthropods were exposed to the fungus for 24 hours and transferred to the base of theplants and canes and were removed after three days. Isolations after a month revealed thatmillipedes and ants were capable of transmitting the fungus onto wounds and causeinfection. Millipede faecal pellets were also evaluated as potential sources of inoculum.Millipedes were fed on Phaeomoniella chlamydospora for 24 hours, surface sterilised andallowed to defaecate in sterile Petri dishes overnight. Faecal material was collected,macerated in water and plated onto potato dextrose agar. Propagules of Phaeomoniellachlamydospora survived passage through the gut of millipedes and were passed out in aviable state to form colonies of Phaeomoniella chlamydospora.This study concludes that a wide variety of arthropods can be a source of inoculumof trunk diseases in vineyards. The results of the dissemination trial provides evidence thatmillipedes and ants are able to disseminate and infect vines with Phaeomoniellachlamydospora. It is therefore, highly likely that other grapevine trunk disease pathogensare transmitted in the same manner. This knowledge highlights the need for control ofcertain arthropods to be taken into consideration when managing grapevine trunk diseasepathogens.

[发布机构] Stellenbosch University