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Reduction of Yield Components by Barley Yellow Dwarf Infection in Susceptible Winter Wheat and Winter Barley in Virginia

1999; Wiley; Volume: 12; Issue: 1 Linguagem: Inglês

10.2134/jpa1999.0105

2689-4114

D. A. Herbert, Erik L. Stromberg, Glenn Franklin Chappell, S. Malone,

Agronomic Practices and Intercropping Systems

Five field studies, during 3 yr (1990–1993) and at two locations, were conducted in eastern Virginia to determine the effect of naturally occurring barley yellow dwarf virus (BYDV) infection on yield components of susceptible 'Barsoy' winter barley (Hordeum vulgare L.) and susceptible 'Wakefield' winter wheat (Triticum aestivum L.) At each study site, yield components were compared for grain harvested from 25 disease loci and 25 adjacent asymptomatic areas. Rhopalosiphum padi L. and Sitobion avenue Fabr. aphid populations were monitored weekly by visual inspection of plant leaves and stems. Aphid numbers were low in all studies, with only one site exceeding the economic threshold of 15 aphids/row ft at any time during the season. The BYDV, PAV strain, was present in all five studies; the MAV strain was present at only one location in 1990–1991. Fall infection was predominant and occurred in all studies, with plants showing typical disease symptoms. Symptoms consistent with spring infection were observed in only one study. With few exceptions, fall infection significantly reduced tiller height, head number, seed number, number of seed per head, 1000 seed weight, and yield. Yield components were not equally affected. Seed number, number of seed per head and yield were reduced the most, by 28%, 22%, and 34%, respectively, compared with only about a 5 to 13% reduction in the other components. Spring infection significantly reduced number of seed per head and 1000 seed weight, but not the other yield components. Research Questions Barley yellow dwarf virus (BYDV) is a recurring plant disease problem in many winter small grain fields in Virginia. What are the yield losses associated with BYDV infection? Economic thresholds for aphids in Virginia small grains are currently set at 15 to 25/row ft in the fall, and 100 to 300/row ft in the spring. Are these thresholds too high, allowing economic loss to occur from BYDV infection transmitted by viruliferous (virus carrying) aphids? Literature Summary Grain aphids are recurring pests of small grains throughout much of the central and eastern USA. The greatest threat to yield reductions associated with aphids (in the eastern states) is the ability of these pests to transmit BYDV, a sporadic but important problem in Virginia winter small grains. Documented yield losses have ranged from 3% to almost 15% in some commercial fields. Crop loss depends on many factors such as the number of aphids, whether the aphids are viruliferous, and when the virus is transmitted in relation to crop growth stage. Low aphid populations in the fall can cause significant crop losses due to fall BYDV infection, while spring BYDV infection has less impact. Because of this, aphid economic thresholds are set at 15 to 25/row ft in the fall, and 100 to 300/row ft in the spring. However, these thresholds may be too high and allowing yield reductions by BYDV infection. New aphid management insecticides are now available to small grain farmers. But they can be expensive and should only be used where BYDV is expected to occur at economically damaging levels. The project reported here was undertaken to gain more knowledge on the impact of BYDV, how to estimate disease loss in fields, and how to use that information in the larger disease management program. Study Description Five field studies, during 3 yr (1990–1993) and at two locations, were conducted in eastern Virginia to determine the effect of naturally occurring BYDV infection on yield components of susceptible 'Barsoy' winter barley and susceptible 'Wakefield' winter wheat. In each study, all wingless aphids in 20 randomly selected 1-ft sections of row were counted at approximately weekly intervals from plant emergence in the fall until early April. At each site, 25 BYDV disease loci (roughly circular area of plants showing disease symptoms), typically 1 to 2 ft in diameter, were marked at around growth stage 26 to 30 (Zadoks growth scale, tillering—early stem elongation). A visually asymptomatic (apparently healthy) locus within 6 ft of each disease locus was also marked. Disease loci were attributed to either fall or spring infection; presence of BYDV was confirmed by an independent lab. The following yield components were compared for grain harvested from all disease and asymptomatic loci: tiller height, number of heads per row foot, seed number per row foot, seed number per head, and 1000 seed weight. Applied Questions Fall aphid thresholds (15–25/row ft) These thresholds only consider wingless aphids. It may take fewer numbers of winged aphids to infect fields with yield reducing levels of BYDV In this study only one site in one season reached the fall economic threshold of 15 aphids/ row ft, yet, BYDV infections were significant in both winter wheat and winter barley in all sites in all three seasons studied. Plants in fall infection loci typically had significantly reduced tiller height, number of heads per row foot, seed number per row foot, seed number per head, 1000 seed weight, and yield compared to asymptomatic plants. Average reductions for seed number per row foot, number of seed per head, and yield were 28%, 22%, and 34%, respectively. Spring infection with BYDV significantly reduced number of seeds per head and 1000 seed weight, but this was based on data from only one site in a single season. How can small grains farmers estimate the effect of BYDV on their fields? First, determine the percentage of area infected by estimating the size and number of disease loci. Then, multiply that percentage by 30% (a conservative estimate of the average percentage yield reduction from BYDV infection) to estimate the potential loss. For example, if 15% of a field had disease loci, we would expect a 4.5% yield loss (0.15 × 0.30 = 0.045). In most years and fields, BYDV symptoms become apparent in the early spring around growth stage 26 to 30 (Zadoks growth scale, tillering—early stem elongation) in late February to early March. Disease loci are roughly circular areas, usually 1 to 2 ft in diameter, and rarely more than 3 ft in diameter. We were better able to spot disease loci from a high vantage point, such as on the top of equipment. Often they are concentrated on field edges, or randomly scattered throughout fields. Typical disease symptoms include the occurrence of stunted, yellow plants, occurring singly or in small groups (loci), located among apparently healthy plants. Older leaves can be prominent and brilliantly yellowed in barley, or yellowish-green in wheat. Management Recommendations Field surveys show that BYDV tends to reoccur in certain areas with some regularity, so fields in those areas appear to be at greater risk to infection. For these areas, we recommend planting after 20 October, and considering the use of a resistant variety such as 'Roane' wheat. We now also recommend making disease severity estimates in the spring and considering implementation of insecticide options the following year in fields in or adjacent to infected fields, especially if planting before 20 October or planting a susceptible variety.