S. J. McKirdy

Use of imidacloprid and newer generation synthetic pyrethroids to control the spread of barley yellow dwarf luteovirus in cereals.

In seven field experiments with wheat and oats sown in autumn, insecticides were applied to control aphids and thereby diminish the spread of aphid-transmitted barley yellow dwarf luteovirus (BYDV). Disease progress was followed over time by enzyme-linked immunosorbent assay (ELISA) on leaf samples using antiserum specific to BYDV serotype PAV. Two foliar applications of either of two newer generation synthetic pyrethroid insecticides, alpha-cypermethrin or beta-cyfluthrin, sprayed before flag leaf emergence and at rates as low as 12.5 g a.i./ha, decreased spread of BYDV by up to 75% and increased grain yields by up to 41%. These pyrethroids were more effective in decreasing BYDV spread than foliar applications of pirimicarb (150 g a.i./ha) or dimethoate (320 g a.i./ha), two applications of which decreased BYDV spread by up to 45% and increased grain yield by up to 14%. Seed treatment with imidacloprid (70 g a.i./ha) delayed BYDV spread in wheat and oats for up to 6 weeks after plant emergence. When imidacloprid seed dressing was followed by two foliar sprays of alpha-cypermethrin, BYDV incidence was decreased by up to 88%, and grain yield was increased by up to 76%. The predominant colonizing aphid species was Rhopalosiphum padi. Dressing seed with imidacloprid and/or foliar applications of the synthetic pyrethroids markedly decreased the numbers of aphids. Numbers colonizing plants were mostly lower than 10 per tiller on nontreated plots, suggesting the grain yield increases resulting from insecticide application were due to control of BYDV rather than to decreased aphid feeding damage. To minimize BYDV-induced grain yield losses in autumn-sown cereals, protection by insecticides should be provided from soon after plant emergence until the twelfth week of plant growth.

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.

Occurrence of Bean Yellow Mosaic Virus in Subterranean Clover Pastures and Perennial Native Legumes

In 1990, infection with bean yellow mosaic virus (BYMV) was widespread in subterranean clover (Trifolium subterraneum) pastures in the south-west of Western Australia. When 100 leaves were sampled at random per pasture, the virus was detected by ELISA in 23 of 87 pastures and incidences of infection ranged from 1 to 64%. BYMV was present in all seven districts surveyed, but highest incidences of infection occurred in the Busselton district. In smaller surveys in 1989 and 1992, incidences of infection in pastures were higher than in 1990, and ranged up to 90%. In 1992, when petals from 1703 samples of 59 species of perennial native legumes from 117 sites were tested by ELISA, only 1% were found infected with BYMV. The infected samples came from 5/7 districts surveyed. Species found infected were Kennedia prostrata, K. coccinea, Hovea elliptica and H. pungens. Representative isolates of BYMV from subterranean clover and native legumes did not infect white clover systemically confirming that clover yellow vein virus (CYVV) was not involved. It was concluded that BYMV infection was present in many subterranean clover pastures, but normally at low incidences, except in epidemic years such as 1992. Also, perennial native legumes are unlikely to act as major reservoirs for reinfection of annual pastures each year. In areas of Australia with Mediterranean climates where perennial pastures are absent, persistence of the virus over summer is therefore by some other method than infection of perennials.

Effect of sowing time on barley yellow dwarf virus infection in wheat: virus incidence and grain yield losses.

In 4 field experiments at 2 sites in Western Australia, wheat was sown at different times in autumn and early winter. Within each sowing, incidence of barley yellow dwarf luteovirus (BYDV) was monitored by ELISA tests on leaf samples, using antiserum specific to BYDV serotype PAV. Delaying sowing decreased BYDV incidence in all 4 experiments. At the higher rainfall site, decreased BYDV incidence due to delayed sowing was associated with increased grain yield and size, and in 1 experiment also with a decrease in the proportion of shrivelled grain. In contrast, at the lower rainfall site, grain yield penalties derived from late sowing negated the yield increases associated with decreased BYDV levels. When imidacloprid seed-dressing followed by foliar sprays of alpha-cypermethrin were applied to control aphids at the lower rainfall site, within each sowing BYDV incidence and the proportion of shrivelled grain decreased and grain yield and size increased. Applications of pirimicarb at the higher rainfall site also decreased BYDV incidence and the proportion of shrivelled grain within each sowing, and grain yield and size were increased. In all experiments, the only, or predominant, colonising aphid species was Rhopalosiphum padi. At flag leaf emergence, delayed sowing decreased the number of aphids on plants. In high BYDV risk zones, when an appropriate insecticide is not applied to control aphid vectors in autumn-sown wheat, delaying sowing may be advisable to minimise BYDV-induced grain yield losses. However, the yield benefits from decreased BYDV infection need to be balanced against possible yield penalties due to late sowing.

Modeling the risk of entry, establishment, spread, containment, and economic impact of Tilletia indica, the cause of Karnal bunt of wheat, using an Australian context

ABSTRACT Modeling techniques were developed to quantify the probability of Tilletia indica entering and establishing in Western Australia (WA), and to simulate spread, containment, and the economic impact of the pathogen. Entry of T. indica is most likely to occur through imports of bulk grain or fertilizer (0.023 +/- 0.017 entries per year and approximately 0.009 +/- 0.009 establishments per year). Entry may also occur through straw goods, new or second-hand agricultural machinery, and on personal effects of travelers who have visited regions with infected plants. The combined probability of entry and establishment of T. indica, for all pathways of entry, is about one entry every 25 years and one establishment every 67 years. Alternatively, sensitivity analysis does show that increases in quarantine funding can reduce the probability of entry to about one entry every 50 years and less than one establishment every 100 years. T. indica is spread efficiently through contaminated farm machinery, seed and soil, rain, air currents, and animals. Depending on the rate of spread of the pathogen and the amount of resources allocated for detection, the time until first detection could range from 4 to 11 years and the economic impact could range from 8 to 24% of the total value of wheat production in WA.

Bean yellow mosaic potyvirus infection of alternative hosts associated with subterranean clover (Trifolium subterraneum and narrow-leafed lupins (Lupinus angustifolius): field screening procedure, relative susceptibility/resistance rankings, seed transmission and persistence between growing seasons

A field screening procedure was devised to determine relative susceptibility and resistance rankings for hosts of bean yellow mosaic potyvirus (BYMV) using BYMV-infected Trifolium subterraneum plants transplanted at eack end of single row test plots. Natural spread of BYMV by aphids resulted in BYMV symptoms in test lines. Four test lines were ranked as highly resistant, nine were resistant, seven were moderately resistant, eight were susceptible and two were highly susceptible to BYMV infection. Disease progress curves plotted for each test line assisted in the ranking process. Relative rankings were independent of flowering date and presence of host alkaloids. Acrythosiphon kondoi, Myzus persicae and Rhopalosiphum padi were the predominant aphid species caught in traps associated with field screening plots. Seven plant species tested were new BYMV host records. Seed of four plant species systemically infected following sap inoculation with BYMV was tested, and seed transmission detected in Melilotus indica (0.5%). When seed of 19 alternative host species that became systemically infected through natural spread was tested, seed transmission was found in Medicago polymorpha (0.9%), Medicago truncatula (0.3%), M. indica (1%), T, arvense (0. 1%), T. campestre (0.2%) and T. glomeratum (0.05%). No seed transmission was detected in T. subterraneum. It is concluded that under broadacre agriculture in the Mediterranean climate of Western Australia, seed-borne infection in naturalized M. polymorpha, T. arvense, T. campestre and T. glomeratum growing in T. subterraneum pastures probably provides the principal means by which BYMV persists over the dry summer to act as primary sources for subsequent spread. The species most likely to contribute to BYMV spread within T. subterraneum pastures and from them to Lupinus angustifolius crops were L. cosentinii, T. campestre, T. dubium and T. subterraneum itself.

Bean yellow mosaic potyvirus infection of alternative annual pasture, forage, and cool season crop legumes : susceptibility, sensitivity, and seed transmission

Seven field and 5 glasshouse experiments were done during 1994-98 to determine the relative susceptibilities and sensitivities of a wide range of alternative annual pasture, forage, and crop legumes to infection with isolate MI of bean yellow mosaic virus (BYMV). Seed harvested from some species was also tested for seed transmission of the virus. Seven of 18 genotypes belonging to 17 species of annual pasture and forage legumes evaluated in 2 replicated field experiments were ranked as highly susceptible to BYMV, 7 as susceptible, 2 as moderately resistant, 1 as resistant, and 1 as highly resistant. The most susceptible and sensitive were Biserrula pelecinus, Trifolium cherleri, T. incarnatum, and T. spumosum. Ornithopus sativus was resistant but sensitive, whereas Hedysarum coronarium was highly resistant. H. coronarium was not infected when manually inoculated repeatedly with 3 different BYMV isolates, Seventy-three of the 94 genotypes of 7 crop legume species tested in the same replicated field experiments were ranked as highly susceptible, including 58/68 of Lens culinaris. Of the remaining genotypes, 6 were susceptible, 5 moderately resistant, 9 resistant, and 1 highly resistant. Five other crop legumes were included in other field experiments in which these species were ranked as highly susceptible (1) or resistant (4). Overall, the most susceptible and sensitive crop legume species were Lens culinaris (most genotypes), Lathyrus cicera, L. ochrus, and Vicia narbonensis. Lathyrus sativus (3 genotypes only), V. sativa (4 genotypes), Cicer arietinum, Pisum sativum, and V. faba were resistant to isolate MI, and Lens culinaris ILL7163 was highly resistant. When infected, C. arietinum was ranked as highly sensitive but symptoms within the other resistant crop species varied in sensitivity between genotypes. Extreme resistance was confirmed in Lens culinaris ILL7163 when it was manually and aphid-inoculated repeatedly with 3 different BYMV isolates. When testing seedlings for seed transmission of BYMV, germination on moist paper towels before testing usually proved more effective than growing in soil in the glasshouse. Low rates of seed transmission of BYMV (0.03-1%) were detected in 9 alternative pasture or forage and 3 alternative crop legume species. This is the first report of seed transmission of BYMV in these species. The pasture or forage species with the highest seed transmission rates were T. clypeatum and T. spumosum (both 1%). The crop legume species in which seed transmission was found were L. cicera (0.1%), L. sativus (0.2%), and V. sativa (0.5%). The high susceptibility and sensitivity to BYMV in some alternative annual pasture, forage, and crop legumes is a cause for concern, especially when they are intended for sowing in BYMV-prone high rainfall zones. Seed transmission of BYMV also leads to inadvertent introduction of the virus to new sites.

Occurrence of barley yellow dwarf viruses in over-summering grasses and cereal crops in Western Australia

During summer and early autumn in 1989, barley yellow dwarf viruses (BYDV) were detected by ELISA in grasses collected at 72 sites in the south-west of Western Australia. Most of the grasses sampled survived the dry conditions in roadside ditches or at the edges of creeks. BYDV was found at ten or more sites in each of five perennial species, Cynodon dactylon, Ehrharta calycina, Eragrostis curvula, Paspalurn dilatatum and Pennisetum clandestinum, and in one summer annual, Digitaria sanguinalis. PAV and RPV were detected at 71% and 86% of infected sites, respectively.During the late winter and early spring in 1989, BYDV was detected in 52% of cereal crops (oats, barley and wheat) sampled in the south coastal region of Western Australia, with >5% infection being recorded in 10% of crops. Highest levels of infection within crops (up to 26%) were found in the Albany and Jerramungup districts. By contrast, the virus was found in only 29% of cereal crops sampled from the western parts of the southern wheat belt where only one crop had >2% infection. When some of the BYDV-infected cereal samples were retested, PAV and RPV were detected in 65% and 39010, respectively.

Occurrence of alfalfa mosaic and subterranean clover red leaf viruses in legume pastures in Western Australia.

When leaf samples were collected from 94 Trifolium subterraneum (subterranean clover) pastures from six districts in spring 1993 in the south-west of Western Australia and tested by enzyme-linked immunosorbent assay, no alfalfa mosaic virus (AMV) or subterranean clover red leaf virus (SCRLV) was detected. In contrast, when 21 irrigated T. repens (white clover) pastures from one district (Bunbury) were sampled and tested in January (summer) 1994, AMV was detected in 16, with eight having infection levels >86%, while SCRLV was found in seven at infection levels of <12%. When a further five T. repens pastures were tested for AMV in October (spring) 1994, the virus was found in all with incidences up to 100%. None of the T. repens pastures with high levels of AMV infection had been resown with T. repens within the last 20 years, whereas those resown within the last five years had little or no infection. AMV was detected in 9/91 annual medic (Medicago spp.) pastures from seven wheatbelt districts sampled in spring 1991 or 1993; a single pasture of M. polymorpha (burr medic) cv. Serena was 21% infected, but the other eight infected ones had <3%. AMV seed transmission was detected in 1/19 commercial seed stocks of M. polymorpha harvested in 1991-93. AMV infection was followed over a 12-year period in M. murex (murex medic) cv. Zodiac seed stocks. It persisted readily through successive seed harvests during this period. It is concluded that infection with AMV and SCRLV is currently not a threat to T. subterraneum pastures in the south-west of Western Australia and that AMV seems not to be one in wheatbelt annual medic pastures provided these are sown with healthy medic seed. In contrast, AMV poses a potential threat to the productivity of irrigated T. repens pastures. SCRLV is also sometimes present in T. repens pastures, but was not found at serious levels.

Infection of alternative hosts associated with annual medics (Medicago spp.) by alfalfa mosaic virus and its persistence between growing seasons.

Under conditions of natural alfalfa mosaic virus (AMV) spread, five plant species found associated with annual Medicago spp. (medics) were infected commonly and another seven sporadically. Ten of these were new records. Because seed of herbaceous plant hosts provides a possible route for virus persistence through dry summer conditions, AMV seed transmission was tested for in alternative hosts. Of ten species systemically infected by sap inoculation with AMV, seed transmission was detected in Melilotus indica (l0%), Ornithopus compressus (0 1%) and Stachys arvensis (2%). Seed of seven naturally infected potential alternative host species was tested, and seed transmission found in Crassula decumbens (0. 1%), M. indica (3%), 0. compressus (0.2%), S. arvensis (0.4%) and Trifolium subterraneum (2%). Carry-over of AMV through seed transmission was detected in seedlings of Hypochaeris glabra (0.2%) and M. indica (0.6-0.8%) that germinated naturally in the field. In grazed, self-regenerated Medicago murex and M. polymorpha swards sown in 1987, the virus persisted for at least seven growing seasons. Levels of infection in M. polymorpha seed produced each year declined, but levels in the general seed bank remained higher due to presence of older seed. It is concluded that under the conditions of broadacre agriculture in the Mediterranean-type climate of Western Australia, seed transmission in C. decumbens, H. glabra and volunteer 0. compressus is an alternative means by which AMV can persist over summer to act as sources of AMV for spread within annual medic pastures. However, persistence through seed of the annual medic cultivars sown and of naturalized annual Medicago species is the principal means of persistence. AMV persists readily from year to year following sowing of infected seed and is likely to cause a recurrent disease problem in annual medic pastures