Pathogenicity of Diatrypaceous Fungi on Grapevines in Australia.

In addition to Eutypa lata, which causes Eutypa dieback, numerous other fungi in the Diatrypaceae family have been isolated from diseased grapevines (Vitis vinifera) and other woody hosts. Pathogenicity trials comprising 70 strains of diatrypaceous fungi representing nine species in six genera were conducted to determine whether these fungi, collected in Australia, were pathogenic to grapevines. When inoculated into wounded trunks of 'Cabernet Sauvignon', eight species, including E. lata, E. leptoplaca, Cryptovalsa ampelina, C. rabenhorstii, Eutypella citricola, E. microtheca, Diatrypella vulgaris, and a Diatrype sp. produced necrotic lesions significantly longer than on controls. In addition, all nine species (including a Cryptosphaeria sp.) were reisolated from the margins of developing lesions and at varying distances above and below the point of inoculation. Diatrypaceous fungi were frequently isolated from asymptomatic or otherwise healthy tissue several centimeters ahead of the disease margin. Methods to control diseases associated with diatrypaceous fungi must take into account their propensity to colonize woody tissues ahead of or in the absence of visible symptoms. Current recommendations for the management of Eutypa dieback using remedial surgery and pruning wound protection appear sufficient for the control of the other diatrypaceous fungi included in this study.

Evaluation of Fungicides for the Management of Botryosphaeria Canker of Grapevines.

The family Botryosphaeriaceae comprises a number of species that are associated with the dieback disease of grapevine (Vitis vinifera), referred to as Botryosphaeria canker. To date, there are few effective agents available for the management of this disease. In this study, fungicides were evaluated for controlling the disease using a combination of in vitro tests and field trials. Twenty fungicides registered for use on other diseases in Australian viticulture were tested in vitro for their effect on mycelial growth of four species within the Botryosphaeriaceae. The concentrations of fungicide at which 50% of mycelial growth is inhibited (EC50 values) were significantly affected both by fungicide and isolate (P < 0.001). Differences in sensitivities of the four species to the fungicides were negligible (0.41 to 0.59 mg/liter). The most effective fungicides were fludioxonil, carbendazim, fluazinam, tebuconazole, flusilazole, penconazole, procymidone, iprodione, myclobutanil, and pyraclostrobin, for which EC50 values were <1.0 mg/liter. These fungicides were evaluated under field conditions, in addition to the pruning wound protectants Bacseal Super, Garrison, and ATCS tree wound dressing, as well as the biological control agent Vinevax. In field trials, carbendazim (Bavistin), fluazinam (Shirlan), tebuconazole (Folicur), Garrison, and ATCS tree wound dressing applied to freshly cut pruning wounds were the most effective and reduced infection by Diplodia seriata and D. mutila by 41 to 65%. These results suggest that the occurrence of Botryosphaeria canker on grapevines may be reduced via treatment of pruning wounds with selected fungicides as soon as possible after pruning.

Plectosporium alismatis comb. nov. a new placement for the Alismataceae pathogen Rhynchosporium alismatis.

The phytopathogen Rhynchosporium alismatis, occurring on Alisma, Sagittaria and other genera in the Alismataceae, is under investigation as a mycoherbicide for alismataceous weeds in Australian rice crops. The type species of Rhynchosporium, R. secalis, belongs in the Helotiales while the phylogenetic relationships of R. alismatis were unknown. To identify fungi related to R. alismatis, the internal transcribed spacer (ITS) region of the ribosomal DNA (ITS1, 5.8S rRNA gene, ITS2) of 56 isolates was sequenced and compared to those available in databases. Analysis of ITS sequences revealed close relationships between R. alismatis and the teleomorph genus Plectosphaerella, as well as several anamorphic fungi which were primarily species of Verticillium. Rhynchosporium alismatis and Plectosphaerella cucumerina clustered together with 98 % bootstrap support. Morphological comparisons supported this relationship indicating that R. alismatis and the anamorphic genus Plectosporium are congeneric. Rhynchosporium alismatis is transferred to Plectosporium, a name proposed for conservation.

Infection process of Plectosporium alismatis on host and non-host species in the Alismataceae.

In Australia, the endemic fungus Plectosporium alismatis (syn. Rhynchosporium alismatis) has potential use as a mycoherbicide for several species in the Alismataceae, a family of aquatic and semi-aquatic marsh herbs, which are considered to be important weeds in rice crops. Of five species identified in south-eastern Australia where rice is grown, two species, Sagittaria graminea and Sagittaria montevidensis are resistant (non-hosts), and no records of P. alismatis on these species have been reported. To better understand the interactions that lead to resistance in these pathosystems, the infection process of the fungus was studied on these species and also on the host Alisma plantago-aquatica, using light, fluorescent and scanning electron microscopy. On all three species both conidial germination and appressorium formation commenced within 6 h of inoculation with greater than 50 % of conidia elongating to form germ tube structures and associated appressoria 12-18 h post inoculation. Germ tube elongation and appressorium formation occurred randomly over the leaf surface. Direct host penetration was facilitated by the production of penetration hyphae that emerged from beneath appressoria. Penetration sites were clearly identified by the presence of spherical holes 0.25-0.5 microm in diam, and were frequently accompanied by resistance reactions in non-host species. Visible symptoms of disease occurred 4-6 d after inoculation of susceptible (host) species.