| Curly top disease (CTD) of vegetable crops is caused by a complex of leafhopper-transmitted viruses in the genus Curtovirus (family Geminiviridae). PCR-based methods were used to identify the curtoviruses involved in CTD of tomatoes in California and to monitor for these viruses in the leafhopper vector, Circulifer tenellus. The predominant species identified were Beet mild curly top virus (BMCTV) and, to a lesser extent, Beet severe curly top virus (BSCTV). PCR detection in leafhoppers indicated that higher amounts of virus, detected in early in the growing season, may be associated with higher incidences of CTD. These assays also revealed geographic locations or ‘hotspots’ having higher proportions of CTV-positive leafhoppers. In contrast to tomato, the importance of CTD in cucurbits is less clear. Cucurbits fields in California and Idaho were surveyed for CTD, and the susceptibility of cucurbits to three curtovirus species was determined by agroinoculation or leafhopper transmission. CTD was not observed in cucurbits in California, but it was detected in cucurbits in Idaho. Low rates of infection and mild to moderate symptoms were observed in cucurbits following agroinoculation with BSCTV or Beet curly top virus (BCTV). In leafhopper transmission experiments, BMCTV infection in cucurbits was positively correlated with the viral titer of the acquisition host. Thus, although cucurbits are relatively poor hosts for CTVs, CTD developed under high virus pressure.;The tissue tropism of BSCTV in Nicotiana benthamiana plants and BMCTV in C. tenellus was investigated by immunolocalization with an affinity-purified capsid protein (CP) antibody. BSCTV was localized to the phloem cells of leaves, stems and flowers of infected plants; and induced hyperplasia and abnormal growth of phloem-associated cells. BMCTV was detected in the digestive tract, hemocoel and the salivary glands of viruliferous leafhoppers. In the digestive tract, virus was localized to the filter chamber and the anterior, mid- and posterior midgut; whereas in the salivary glands, it was detected in the principal salivary glands. A non-transmissible CP mutant, CP25-28, was detected in the digestive tract and hemocoel but not in the salivary glands, revealing a barrier for virus transmission. |
