Impact of seed-applied pesticides on rhizobial survival and legume nodulation.

AIMS: Compatibility of seed-applied pesticides and rhizobial inoculants is an important consideration for farmers when sowing legumes. Some of the seed-applied pesticides may influence rhizobial growth and nodulation, but there is currently little available information on the potential inhibitory effects. Therefore, common seed fungicidal and insecticidal treatments were assessed to determine adverse impacts on rhizobial inoculants both in vitro, on treated seed, and in the field. METHODS AND RESULTS: Initially, the in vitro toxicity of the seed-applied fungicides Thiram 600, P-Pickel T (PPT), their active ingredients (thiram and thiabendazole) and the insecticide Gaucho to rhizobia was measured with filter discs containing varying concentrations of the pesticides. Pea and chickpea seed was then coated with the same pesticides and inoculated with rhizobia in different inoculant substrates to determine bacterial survival and nodulation. Finally, a field trial using the fungicide PPT and commercial inoculants was conducted. Some seed fungicide treatments were found to be inhibitory to rhizobia and reduce nodulation under monoxenic conditions and in the field. SIGNIFICANCE AND IMPACT OF THE STUDY: These data provide more detailed information on the compatibility of specific rhizobial inoculants with common seed-applied pesticides. This research will provide information on the compatibility of rhizobia and seed-applied pesticides, and assist farmers to select sowing practices which reduce the risk of crop nodulation failures.

Genetic Characterization and Diversity of Rathayibacter toxicus.

ABSTRACT Rathayibacter toxicus is a nematode-vectored gram-positive bacterium responsible for a gumming disease of grasses and production of a highly potent animal and human toxin that is often fatal to livestock and has a history of occurring in unexpected circumstances. DNA of 22 strains of R. toxicus from Australia were characterized using amplified fragment length polymorphism (AFLP) and pulsed-field gel electrophoresis (PFGE). AFLP analysis grouped the 22 strains into three genetic clusters that correspond to their geographic origin. The mean similarity between the three clusters was 85 to 86%. PFGE analysis generated three different banding patterns that enabled typing the strains into three genotypic groups corresponding to the same AFLP clusters. The similarity coefficient was 63 to 81% for XbaI and 79 to 84% for SpeI. AFLP and PFGE analyses exhibited an analogous level of discriminatory power and produced congruent results. PFGE analysis indicated that the R. toxicus genome was represented by a single linear chromosome, estimated to be 2.214 to 2.301 Mb. No plasmids were detected.

Inducible Flavone in Oats (Avena sativa) Is a Novel Defense Against Plant-Parasitic Nematodes.

ABSTRACT The induction of defense compounds in oats (Avena sativa) in response to invasion by parasitic nematodes and to application of the wound hormone methyl jasmonate was examined. Oats cv. Quoll seedlings were challenged with Pratylenchus neglectus, Heterodera avenae, and Ditylenchus dipsaci and treated with 1 x 10(-4) M methyl jasmonate. Three compounds, isolated in methanolic root and shoot extracts of oats, exhibiting an absorbance spectrum typical of flavone glycosides, were induced by nematode invasion and methyl jasmonate. These were identified as flavone-C-glycosides by mass spectrometry. The effect of the flavone-C-glycosides on the invasion by and development of cereal cyst nematode H. avenae was assessed using methanolic extracts of shoots and roots from methyl jasmonate-treated plants. Both extracts impaired nematode invasion and development. When the extracts were fractionated by high voltage paper electrophoresis, only one flavone-C-glycoside, O-methyl-apigenin-C-deoxyhexoside-O-hexoside, inhibited nematode invasion. The protective effect of the induction of flavone-C-glycosides in oats by methyl jasmonate was evaluated against H. avenae and P. neglectus. Treatment with methyl jasmonate reduced invasion of both nematodes and increased plant mass, compensating for damage caused by the nematodes, and is attributed to the active flavone-C-glycoside. The active compound, O-methyl-apigenin-C-deoxyhexoside-O-hexoside, has not been implicated previously in plant defense against any pest or pathogen, and appears to provide protection against the major cereal nematodes Heterodera and Pratylenchus.

First Report of Grapevine Downy Mildew (Plasmopara viticola) in Commercial Viticulture in Western Australia.

Despite the suitability of climate, Western Australia was one of the few grape (Vitis vinifera L.) growing areas free of grapevine downy mildew (Plasmopara viticola (Berk. & M. A. Curtis) Berl. & De Toni in Sacc.). Area freedom had been maintained by restricting the movement of host material and machinery from outside the state and fungicide use in Western Australia vineyards had been considerably less. P. viticola was detected in 1997 in 14 of 15 vines growing at Kalumburu, a remote community in the semi-arid tropics of Western Australia, and was eradicated. In October 1998, grape leaves with oilspots typical of downy mildew were received from a grower in the Swan Valley near Perth, one of the main production areas of Western Australia. Sporangia were hyaline and ellipsoid (14 × 11 µm), were borne on treelike sporangiophores, and were consistent with those described for P. viticola (1). This is the first record of P. viticola in commercial viticulture in Western Australia. A response plan for exotic diseases was activated and after 2 weeks of surveillance the disease was found in 45 of 70 vineyards surveyed of the 280 vineyards in the Swan Valley. Given the extent of spread, eradication of downy mildew was not considered possible. Weather data for August to October 1998 indicated the likelihood of several infection periods from budburst to flowering when the disease was first detected. Crop loss will be considerable in many vineyards. P. viticola was also found in bench-grafted cuttings in pots in leaf consigned from the Swan Valley to several other areas in August 1998. Downy mildew was found in other areas only in association with these consigned vines. An industry code of practice, including hygiene, is being developed to slow the rate of spread of P. viticola in Western Australia. Reference: (1) Anon. C.M.I. Descriptions of Pathogenic Fungi and Bacteria No. 980, 1989.

Sampling ryegrass to assess the risk of annual ryegrass toxicity.

Most stock losses caused by annual ryegrass toxicity occur because stockowners unknowingly allow their stock to graze annual ryegrass (Lolium rigidum) infected with the bacterium Clavibacter toxicus. To help stockowners avoid losses we have developed criteria for a testing service to determine the risk of poisoning before the pasture is grazed. Low, medium and high risk categories were selected using samples of dry, mature ryegrass seedheads collected by stockowners from untreated, infected pastures in South Australia. The proportion of toxic paddocks in each risk category over all the seasons tested was 11%, 32% and 76%, respectively, and these accounted for 7%, 14% and 79% of total stock losses. The proportion of paddocks in which stock were poisoned did not vary significantly between years, was not affected by variation in sample weight, and did not vary between South Australia and Western Australia.