Vector Relations Confirmation of Polymyxa graminis as a Vector of Wheat Spindle Streak Mosaic Virus

SLYKHUIS, J. T., and D. J. S. BARR. 1978. Confirmation of Polymyxa graminis as a vector of wheat spindle streak mosaic virus. Phytopathology 68: 639-643. Wheat spindle streak mosaic virus was transmitted via isolates of each of the zoosporic fungi were cultured on wheat water from soil-free roots of wheat plants infected by growing infected with the virus by sap inoculation of leaves, the virus in infective soil, but not from plants grown in sterile soil and was transmitted by root association in sand only from plants infected with the virus by sap inoculation of the leaves. The infected with P. graminis. The optimum temperature for zoosporic fungi Lagena radicicola, Olpidium brassicae, transmission of the virus in soil, 15 C, is above the optimum Polymyxa graminis, Rhizophydium graminis, and Pythium for the development of the virus, 10 C, but within the lower spp. usually were present in the roots of plants infected from range of temperatures favorable for the development of P. soil, and in the roots of most plants in 10 sets infected graminis. successively by root association in sterilized sand. When Wheat spindle streak mosaic (WSSM) is widespread in transmission, and to determine if any of these fungi some years in most fields in areas of southern Ontario transmitted WSSMV. where winter wheat is grown most intensively (10). It has been reported in several central and northeastern States MATERIALS AND METHODS of the USA, in France, and recently in India (1, 14). It is Soil for most of the experiments reported here was a caused by a soil-borne virus, WSSMV, with very long, sandy loam collected at the Central Experimental Farm, filamentous particles, quite distinct from the short rodOttawa, from a field in which winter wheat had been like particles of soil-borne wheat mosaic virus (WSBMV) grown every 3rd yr starting in 1952. By 1967, plants with (4), which has not been found in Ontario and was not symptoms of WSSM were observed in this field. detected in any of the plants grown in soils used in these Subsequently winter wheat was planted each year to build experiments. up and maintain a high level of infectivity. The soil was Because a zoosporic fungus, Polymyxa graminis Led. collected in October, dried on a greenhouse bench, sifted has been associated with the transmission of WSBMV (4, to break down the lumps, and stored dry in a greenhouse 9), this fungus has been a prime suspect as a vector of where temperatures ranged from about 15 to 30 C. WSSMV. Although P. graminis usually has been found Winter wheat (Triticum aestivum 'Kent') was used as in the roots of wheat plants infected with WSSMV the test host and for multiplying the virus and culturing wherever the disease occurs, other fungi that infect wheat the zoosporic fungi. Wheat was infected from soil by roots in similar ways also generally are present, hence sowing 10-15 seeds in 7.5-cm diameter pots of the test soil, they also were suspect as vectors. One of these, Olpidium watering moderately, and growing at 15 C with about brassicae (Wor.) Dang., is a vector of several other viruses 12,000 lux of light for 12 hr per day, genereally for 21 (7). The other suspects included Rhizophydium graminis days, but for some tests for 14 days and for others 28 days. Led., Lagena radicicola Vanterpool and Led., and several To terminate contact with the infective soil, and also to Phythium species (2, 3, 6, 8). stimulate disease development (13), the young plants were Attempts to determine a specific vector or vectors from removed from the infective soil, their roots were washed among these were frustrated by slow rates and low levels thoroughly with running tap water, then they were reof transmission (10), specific temperature requirements potted in sterile soil or sand, watered with half-strength (9), slow development of symptoms and difficulties in Hoagland's nutrient solution, and grown at 10 C. obtaining pure isolates of the zoosporic fungi (2), most of Symptoms usually developed in 60 to 90 days from which have not been cultured on artificial media, planting. Roots were examined for the presence of The experiments reported here were to indicate some of zoosporic fungi after the diseased plants were grown in the specific characteristics and requirements for vector sand at 15 C or 20 C for an additional 3-4 wk. Generally, 00032-949X/78/000 105$03.00/0 by this time resting spores were found easily Copyright © 1978 The American Phytopathological Society, 3340 microscopically (2). A phase-contrast microscope was Pilot Knob Road, St. Paul, MN 55121. All rights reserved, used particularly when searching for zoosporangia.