Both susceptible (left) and resistant plants are infected with BYD, even though only the susceptible variety shows symptoms. Corn (right) has been implicated as an important yellow dwarf of source of BYD virus in some areas of the United States. California cereals in two days. This method works well in detecting BYDVs in most plant species, and is used when large numbers of samples are John A. Griesbach a Rebecca Creamer a Gale Lorens to be screened for BYDV types. Bryce W. Falk a Calvin 0. Qualset a Lee F. Jackson The newest method for BYDV identifica- tion involves cloned recombinant cDNA probes. cDNA probe analysis of plant ex- Research on barley yellow dwarf reduced yields. The symptoms often aren’t tracts is fast and can be used to process distinct, however, and they can be con- large numbers of samples, but the tech- disease in California small grains fused with nutrient deficiencies or other nique is complex and expensive. is designed to develop cultural conditions. Infection by more than one We have used a recombinant DNA probe controls and cultivars with im- type of barley yellow dwarf virus can in- to measure responses of various proved resistance. Recent discov- crease disease severity and crop losses. cultivars to infection by PAV types of the ery of BYDV satellite RNA that in- BYDVs are transmitted by a variety of virus. The probe (pPA8, developed in cereal aphids, which acquire the virus by Australia) is specific for PAV. Developing hibits replication of the virus feeding on infected plants. Primary infec- and using recombinant DNA probes may offers additional hope of control. tion (field-to-field or long-distance spread) prove helpful in screening cultivars for re- is initiated by migrating winged aphids in sistance, since they directly measure viral Much has been learned about barley yellow the late fall or spring. Secondary infections replication. dwarf since it was discovered in 1951 that (within-field spread) are initiated by Distribution in California aphid-transmitted viruses (BYDVs) caused winged or nonwinged aphids moving virus the disease. Despite its rating as the most from initial infection points. Once an aphid A primary objective of our work has been economically important viral disease of acquires BYDV, it becomes a carrier for life. to learn more about the seasonal, geo- small grains in the world, however, infor- graphical, and host incidence of different mation on how to deal with BYD is still Detection and diagnosis BYDVs in California. Analysis of 561 limited. We use three methods to identify specific samples of cereal grains from 16 locations BYD affects all sinall grains, many pasture BYDVs in California: aphid transmission, in 1987 showed that BYDVs occur through- grasses and grassy weeds, corn, and . serology, and nucleic acid hybridization. out the state, but that the major growing Although California hasn’t had a major epi- The first, allowing aphids to acquire the regions vary in the relative frequency of demic of the disease in many years, local virus by feeding on infected plants and BYDV types (table 1). PAV, the predomi- epidemics are commonplace in many parts transferring them to healthy plants, is time- nant type in California, ranged in 1987 from of the world. In the United States losses to consuming and requires large aphid-free a high of 90.5% of BYDV in the Imperial BYD are estimated at 1% to 3% annually, greenhouses and aphid rearing facilities. Valley to a low of 49.0% in coastal regions. but up to 40% damage has been reported in Specific diagnosis of a given BYDV may MAVs were most common in the San isolated cases. Mild, wet winter weather take several months and repeated transfers. Joaquin and coastal regions, and lowest in followed by a dry spring can lead to a However, aphid transmission assays are the Imperial Valley. RPVs were the least population explosion of aphid vectors that essential because they identify which aphid common type in all growing regions, and infest grain fields when the plants are species actively transmits a given BYDV. were not detected in the Imperial Valley. young and most vulnerable. The most common assay method is a se- The highest levels of RPVs were found in Symptoms of infection generally include rological technique called ELISA (enzyme- the coastal region and Sacramento Valley. reddening of oat and yellowing of barley linked immunosorbent assay). With ELISA, Mixed infections of two and three BYDV and wheat leaves, stunting of plants, and the specific type of BYDV can be identified types occurred infrequently except in the

CALIFORNIA AGRICULTURE, JANUARY-FEBRUARY 1989 23 San Joaquin Valley (table 1). Five of six or wheat, even though much of the to control virus-carrying aphids. While locations sampled there had higher than world collection has been screened. Resis- insecticides won’t eradicate aphids, they expected levels of MAV-PAV dual infec- tance in oats and wheat appears to be will suppress secondary spread and in ef- tions. Of the 135 samples taken, 49 were quantitative and controlled by many genes. fect delay infection, thereby limiting crop ELISA-positive for this combination. A 1988 Resistant oat cultivars were developed us- losses. survey will give us a second year of data to ing several sources of HPR. Highly resistant Aphids are being monitored in a coordi- see if dual infections are a regular phe- wheat, however, has not been found even nated project in the western United States nomenon in the San Joaquin Valley. from several thousand cultivars in the world and in England to determine when cereal Most research indicates that cross-protec- collection. Several with limited resistance, aphid vectors migrate. Monitoring data tion does not occur in dual infections; such as Anza, are being intercrossed to se- from Washington state during the fall and rather, they have an additive or synergistic lect for highly resistant types. winter of 1982-83 showed that 3.4% to effect on severity of the disease. We are The UC Davis program to increase resis- 14.5% of the aphids trapped transmitted testing these California types individually tance is multifaceted. The U.S. Department BYDV, and that oat-bird cherry aphid was and together for yield losses in the main of Agriculture supports a joint program in the most important vector. This informa- cereal cultivars grown in the San Joaquin which all entries from the world collection tion can be used to adjust planting dates or Valley. If a unique BYD problem exists in of barley, oats, and wheat are being schedule insecticide sprays. this area, a management plan based on screened at Davis for resistance to BYDV. An aphid monitoring station was recently statewide averages may not be the best Cooperative exchange of material with re- placed on the UC Davis campus. Aphids strategy to control this BYD damage. searchers throughout the United States and are collected weekly and identified by spe- We have detected BYDV in about 25% of many parts of the world allows access to cies. In a parallel study, weekly field plant- 378 samples of wild oats, native grasses, many sources of resistance. A novel ap- ings of trap plants (oats) are synchronized permanent pasture, and corn collected proach at Davis to improve resistance in with aphid collection to detect the onset throughout the state. The data are from wheat is the transfer of the barley chromo- and duration of fights of BYDV-viruliferous ELISA tests; aphid transmission tests have some containing the Yd, gene. aphids and to identify the specific types not been completed. It seems, however, The third BYD control strategy, the use of they transmit. The correlation between that BYDVs can survive in wild and native insecticides to control aphids, is usually not disease onset and a particular vector could grasses and other alternate hosts while economically practical and has not been be valuable in management decisions. small grains are temporarily absent. tested adequately in California. Pyrethroids Future research Control are used in Europe on spring-sown grains From our research we hope to gain a de- BYD is controlled by three main methods: tailed understanding of BYD in California restricting the planting date, using resistant cereals. The goal is to develop more effec- crop plants, and applying insecticides to tive ways of controlling BYD by cultural control aphids. methods as well as through the use of ce- Restricting the planting date ensures that real cultivars with improved resistance to plants are not in the susceptible seedling BYDV types. stage during peak aphid flights. The age of The discovery by Australian plant virolo- the plant at infection is a critical factor in gists of BYDV satellite RNA (a biological crop losses. Yield loss is about twice as agent smaller than the BYDV virus particle great if plants are infected at the seedling that is parasitic on the RPV type) is a posi- stage rather than the tillering stage. Simi- tive development. Satellites are fairly com- larly, plants infected at the tillering stage mon among plant viruses, but whether or will suffer about twice the loss of those in- not they offer a realistic means of virus fected at stem extension and pre-boot. suppression is just being investigated. The The recommended planting time of No- procedure involves the insertion of the sat- vember to December for most of California ellite genetic sequence into a susceptible allows the grain to be in a fairly advanced cultivar. When the cultivar becomes in- state when aphid populations and flights fected with BYDV, the BYDV replication is peak in March and April. However, grow- inhibited by the satellite RNA and thus is ers can be forced into late planting dates by controlled. This protection, if realized, has unfavorable weather or other factors, which the potential to by-pass the control of aphid will increase the risk of exposure to BYD. vectors and the restrictions of planting Increased vulnerability could also result dates to avoid aphids. from earlier planting (September-October), which may be necessary with winter that require vernalization. Such John A. Griesbach is Post-Graduate re- seedlings may be exposed to aphids and searcher, and Rebecca Creamer is Gradu- BYD in the fall before the onset of cooler ate student, Department of Plant Pathology, temperatures. University of California, Davis; Gale Lorens The most effective way to control BYD is is Post-Doctoral Associate, U.S.Department through host plant resistance (HPR). A of Agriculture Agricultural Research Seru- single gene for BYD resistance in barley ice, Albany, California; Bryce W. Falk is (Yd,), has proved effective in reducing Associate Professor, Department of Plant losses to the most common BYDV type Pathology, UC Davis; Calvin 0.Qualset is (PAV). Most barley cultivars carry the Yd, Professor, and Lee F. Jackson is Extension gene. A single gene with the same degree Agronomist, Department of Agronomy and of resistance hasn’t been identified in either Range Science, UC Davis.

24 CALIFORNIA AGRICULTURE, JANUARY-FEBRUARY 1989