ABSTRACT

The feeding periods of the grain aphid Sitobion avenae an important vector of barley yellow dwarf virus (BYDV), was studied in six varieties of spring barley (Hordeum vulgare) in an attempt to explain the mechanisms in some varieties apparently conferring resistance/tolerance to BYDV. There was no significant difference between the apparently resistant and susceptible varieties as they affect aphid feeding period. The mechanisms underlying BYDV resistance do not seem to involve factors that invoke alterations in the feeding periods. Suggestions for future studies are highlighted.

INTRODUCTION

Barley yellow dwarf virus (BYDV) is found worldwide having been identified in at least one country in every continent. It is the most widely distributed of all the cereal viruses (Rochow, 1970; Lister & Ranieri, 1995). Economic losses have been associated with BYDV in the U.S.A. (Oswald & Houston, 1953), U.K. (A’Brook, 1974), and New Zealand (Smith, 1993).

In the U.K., the grain aphid Sitobion avenae, the rose grain aphid Metopolophium dirhodum and the bird cherry aphid Rhopalosiphum padi are the most important BYDV vectors (A’Brook & Dewar, 1980). In spring-sown cereals S. avenae is considered to be the principal source of primary infection to the crop (Plumb, 1977). Little is known regarding aphid feeding as a component of BYDV resistance despite the fact that resistance in cereals is related to aphid activity (Ullman et al., 1988).

It is thought that early sowing reduces aphid colonisation prior to stem elongation and hence decreases damage from virus but even this may not be adequate after very mild winters when aphids begin to fly into the crop early in the season. The use of insecticidal applications to control BYDV in spring-sown cereals has been attempted with little success in the U.K. (Irwin & Thresh, 1986). In order to assess the potential of some fortuitously discovered resistant varieties (Collins, pers. com.) it is essential to understand the mechanism by which that resistance is conferred. This study sought to show whether or not effects on inoculation access feeding periods (IAFPs) are responsible for the apparent BYDV resistance in three varieties of spring barley.

MATERIALS AND METHODS

Test plants used in this study were 3 BYDV "resistant" (Cooper, R1; Optic, R2; and Amber, R3) and 3 susceptible (Alexis, S1; Derkado, S2; and Triumph, S3) varieties of spring barley. Oat (cv. Dula) was used as the control since it is thought to be the most susceptible cereal to BYDV. Ten seeds of each variety were planted. One set was used for the four inoculation access feeding period (IAFP) used. One week old test plants were used and one winged viruliferous S. avenae per plant for the required length of time. Aphids were contained in a 2.5cm diameter cellulose acetate tube with a netting top. After each IAFP each aphid was removed and any nymphs killed. The IAFP used were 6, 12, 24 and 48 hours (Lowles et al., 1996). The experiment was replicated six times. Photoperiod and temperature were not controlled in the glasshouse and the mean temperature was 21^oC (the maximum 26^oC and the minimum 16^oC) during this study. The indirect double antibody sandwich ELISA was used in testing the plants for BYDV presence three weeks after inoculation.

RESULTS AND DISCUSSION

ELISA test results are summarised in Table 1. There was no significant relationship between;

1. The proportion of plants with virus and inoculation access feeding period (6hr, 12hr, 24hr and 48hr) hereafter referred to as TIME
2. resistance groupings
   1. Oats           Vs
   2. S1 + S2 + S3      Vs
   3. R1 + R2 + R3      hereafter referred to as F1
3. resistance groupings
   1. Oats           Vs
   2. S1           Vs
   3. S2           Vs
   4. S3           Vs
   5. R1 + R2 + R3           hereafter referred to as F2
4. resistance groupings
   1. Oats           Vs
   2. S1           Vs
   3. S2           Vs
   4. S3           Vs
   5. R1           Vs
   6. R2           Vs
   7. R3           hereafter referred to as F3
5. TIME and F1
6. TIME, F1 and F2
7. TIME, F1, F2 and F3

Table 1: Accumulated analysis of deviance: Proportion of infected plants and feeding periods as shown by ELISA

change	df	deviance	mean deviance	deviance ratio
+ TIME	3	10.311	3.437	1.26NS
+ F1	2	6.870	3.435	1.26NS
+F1. F2	2	4.58	2.293	0.84NS
+ F1. F2. F3	2	0.298	0.149	n 0.05NS
+ TIME . F1	6	4.523	0.754	0.28NS
+ TIME . F1. F2	6	3.919	0.653	0.24NS
+TIME . F1. F2. F3	6	3.727	0.621	0.23NS
Residual	140	381.539	2.725
Total	167	415.773	2.490

NS = Not statistically significant

The data in this experiment did not show that plant resistance to aphids and possibly BYDV transmission is associated with an increase in the time required by aphids to penetrate the phloem. The results also did not show any difficulty in getting any of the varieties infected with the virus as ELISA test results showed that the virus gets into all the plants. In general, when Oats, all susceptible varieties and all "resistant" varieties were compared, it was observed that the "resistant" varieties had lower but not significantly different infection levels in comparison to the others at all the inoculation access feeding periods used. This indicates that host effect on feeding period cannot adequately explain the mechanism behind this resistance. It is suggested that other aspects of aphid feeding (e.g. depth of probing and rate of probing) should be studied. Tolerance (host-virus interaction) seems unlikely because even with the "resistant" varieties, some plants showed symptoms, and these symptoms are as strong as those in "susceptible" varieties. Also ELISA titre values (data not shown) gave no such indication of the possibility of tolerance in these spring-sown varieties of barley. In autumn sown barley the tolerance to BYDV is known to be governed by a Yd2 gene.

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