Potato spindle tuber viroid: Stability on Common Surfaces and Inactivation With Disinfectants.

The length of time Potato spindle tuber viroid (PSTVd) remained infective in extracted tomato leaf sap on common surfaces and the effectiveness of disinfectants against it were investigated. When sap from PSTVd-infected tomato leaves was applied to eight common surfaces (cotton, wood, rubber tire, leather, metal, plastic, human skin, and string) and left for various periods of time (5 min to 24 h) before rehydrating the surface and rubbing onto healthy tomato plants, PSTVd remained infective for 24 h on all surfaces except human skin. It survived best on leather, plastic, and string. It survived less well after 6 h on wood, cotton, and rubber and after 60 min on metal. On human skin, PSTVd remained infective for only 30 min. In general, rubbing surfaces contaminated with dried infective sap directly onto leaves caused less infection than when the sap was rehydrated with distilled water but overall results were similar. The effectiveness of five disinfectant agents at inactivating PSTVd in sap extracts was investigated by adding them to sap from PSTVd-infected leaves before rubbing the treated sap onto leaves of healthy tomato plants. Of the disinfectants tested, 20% nonfat dried skim milk and a 1:4 dilution of household bleach (active ingredient sodium hypochlorite) were the most effective at inactivating PSTVd infectivity in infective sap. When reverse-transcription polymerase chain reaction was used to test the activity of the five disinfectants against PSTVd in infective sap, it detected PSTVd in all instances except in sap treated with 20% nonfat dried skim milk. This study highlights the stability of PSTVd in infective sap and the critical importance of utilizing hygiene practices such as decontamination of clothing, tools, and machinery, along with other control measures, to ensure effective management of PSTVd and, wherever possible, its elimination in solanaceous crops.

Quantification of Yield Losses Caused by Barley yellow dwarf virus in Wheat and Oats.

Grain yield data obtained from five field experiments in Western Australia from 1992 to 1994, in which insecticide applications suppressed the spread of Barley yellow dwarf virus (BYDV) in wheat and oats, were used to quantify the relationships between incidence of BYDV and yield gaps, 500-seed weight, and percent shriveled grain. Yield gaps ranged from 0 to 2,700 kg/ha, and the relationship between yield gap and incidence of BYDV was always linear. Single point yield loss models revealed that BYDV infection explained most of the variation in yield gaps. There was a significant linear relationship between incidence of BYDV and 500-seed weight for wheat, but not for oats. The percent shriveled grain always increased with an increase in incidence of BYDV in wheat but not in oats. Cost-benefit relationships were determined for the return on investment when deploying imidacloprid-treated seed and/or one or two foliar applications of pyrethroid insecticides to reduce incidence of BYDV and to decrease the yield gaps in wheat and oats due to BYDV.

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.