Epidemiology of Blackberry yellow vein associated virus.

Blackberry yellow vein disease is one of the most important diseases of blackberry in the United States. Several viruses are found associated with the symptomology but Blackberry yellow vein associated virus (BYVaV) appears to be the most prevalent of all, leading to the need for a better understanding of its epidemiology. Efficient detection protocols were developed using end-point and quantitative reverse-transcription polymerase chain reaction. A multi-state survey was performed on wild and cultivated blackberry to assess the geographical distribution of the virus. Two whitefly species, Trialeurodes abutilonea and T. vaporariorum, were identified as vectors and 25 plant species were tested as potential BYVaV hosts. The information obtained in this study can be used at multiple levels to better understand and control blackberry yellow vein disease.

A New Expanded Host Range of Cucurbit yellow stunting disorder virus Includes Three Agricultural Crops.

Cucurbit yellow stunting disorder virus (CYSDV) was identified in the fall of 2006 affecting cucurbit production in the southwestern United States (California, Arizona), as well as in nearby Sonora, Mexico, resulting in nearly universal infection of fall melon crops in 2006 and 2007, and late infection of 2007 spring melons. Survival of CYSDV through the largely cucurbit-free winter months suggested the presence of weed or alternate crop hosts, although previous studies indicated a limited host range restricted to members of the Cucurbitaceae. To determine potential reservoir hosts for CYSDV in desert production, weed and crop hosts were collected from throughout the region over a period of 26 months, and were tested for the presence of CYSDV by reverse transcription-polymerase chain reaction (RT-PCR) using CYSDV HSP70h- and coat protein gene-specific primers. Many noncucurbits collected from infected melon fields and nearby areas were symptomless and virus free; however, CYSDV was detected in alfalfa (Medicago sativa), lettuce (Lactuca sativa), and snap bean (Phaseolus vulgaris), as well as in several weed species widely prevalent in the region. Typical crinivirus symptoms of interveinal yellowing and leaf brittleness were observed on CYSDV-infected snap bean, alkali mallow (Sida hederacea) and Wright's groundcherry (Physalis wrightii), while other infected crop and weed hosts were symptomless. Transmission tests demonstrated that lettuce, snap bean, alkali mallow, Wright's groundcherry, and buffalo gourd (Cucurbita foetidissima) could serve as virus reservoir hosts for transmission of CYSDV to melon and other cucurbits. These results expand the previously known host range of CYSDV, demonstrating that the virus is capable of infecting not only members of the Cucurbitaceae, but also plants in seven additional taxonomic families.

Co-infection by two criniviruses alters accumulation of each virus in a host-specific manner and influences efficiency of virus transmission.

Tomato chlorosis virus (ToCV), and Tomato infectious chlorosis virus (TICV), family Closteroviridae, genus Crinivirus, cause interveinal chlorosis, leaf brittleness, and limited necrotic flecking or bronzing on tomato leaves. Both viruses cause a decline in plant vigor and reduce fruit yield, and are emerging as serious production problems for field and greenhouse tomato growers in many parts of the world. The viruses have been found together in tomato, indicating that infection by one Crinivirus sp. does not prevent infection by a second. Transmission efficiency and virus persistence in the vector varies significantly among the four different whitefly vectors of ToCV; Bemisia tabaci biotypes A and B, Trialeurodes abutilonea, and T. vaporariorum. Only T. vaporariorum can transmit TICV. In order to elucidate the effects of co-infection on Crinivirus sp. accumulation and transmission efficiency, we established Physalis wrightii and Nicotiana benthamiana source plants, containing either TICV or ToCV alone or both viruses together. Vectors were allowed to feed separately on all virus sources, as well as virus-free plants, then were transferred to young plants of both host species. Plants were tested by quantitative reverse-transcription polymerase chain reaction, and results indicated host-specific differences in accumulation by TICV and ToCV and alteration of accumulation patterns during co-infection compared with single infection. In N. benthamiana, TICV titers increased during co-infection compared with levels in single infection, while ToCV titers decreased. However, in P. wrightii, titers of both TICV and ToCV decreased during mixed infection compared with single infection, although to different degrees. Vector transmission efficiency of both viruses corresponded with virus concentration in the host in both single and mixed infections. This illustrates that Crinivirus epidemiology is impacted not only by vector transmission specificity and incidence of hosts but also by interactions between viruses and efficiency of accumulation in host plants.

Epidemiology of Strawberry pallidosis-associated virus and Occurrence of Pallidosis Disease in North America.

Strawberry pallidosis-associated virus (SPaV) was found closely associated with pallidosis disease. The modes of transmission of the virus were studied, including pollen, seed (achene), and whitefly transmission. Three whitefly species were tested for their ability to transmit SPaV, but only the greenhouse whitefly, Trialeurodes vaporariorum, was identified as a vector of the virus. Testing strawberries for SPaV and Beet pseudo yellows virus (BPYV), a second crinivirus associated with pallidosis disease, in strawberry-producing areas in North America confirmed a high incidence of both viruses in areas where high populations of whiteflies were present. Infection rates as high as 90% for SPaV and 60% for BPYV were observed when plants exhibiting decline symptoms were tested. Lower rates of infection were found in regions where whiteflies were absent or found in low numbers. The role of these criniviruses in the strawberry decline observed over the past few years along the western coast of North America was examined.