Search

Beet leafhoppers were collected weekly on yellow sticky traps placed at 36 locations in Morrow and Umatilla Counties in northeastern Oregon in April through November 2007, 2008, and 2009. Insects were counted, collected, and a subset of the insects was tested for the presence of the beet leafhopper-transmitted virescence agent phytoplasma, the causal agent of potato purple top disease in this region. Beet leafhoppers were present throughout the sampling period and the number of insects collected peaked in June of each year with smaller peaks in July and October. Of the 804 insects tested for phytoplasma in 2007, 2008, and 2009, 21, 18, and 22% tested positive for phytoplasma, respectively. Most of the phytoplasma-positive insects were collected from mid-June through July. Positive insects, however, were collected as late as 13 November in 2007 and 2008. These data indicate that a relatively high proportion of the beet leafhoppers in this area are harboring the phytoplasma. Therefore, the potential for development of purple top disease of potatoes from migrating beet leafhoppers in this important potato producing region is quite high and measures to control this pest throughout the growing season are probably necessary in order to reduce disease pressure.

Root galls possibly caused by Spongospora subterranea were observed on hairy nightshade (Solanum sarrachoide; HNS). HNS galls and galls from potato were used to artificially inoculate potato and HNS. Eighty-three and 52% potato and HNS plants inoculated with potato inoculum had root galls, respectively. Ten and 31% potato and HNS plants inoculated with HNS inoculum had root galls, respectively. A S. subterranea-specific PCR significantly correlated (P < 0.0001, Phi Coefficient = 0.69) with visual assessment of gall incidence in 34 of 40 samples. Five of 40 samples had no galls, but gave positive PCRs. One of 40 samples had root galls, but gave a negative PCR. More sporosori were produced on potato than on HNS. Therefore, the root galls on HNS were caused by S. subterranea that is able to complete its life cycle on HNS and produce new generations of sporosori that are infectious on potato.

The ability of several weed species to serve as hosts for tobacco rattle virus (TKV), the causal agent of corky ringspot disease of potato (CRS), and its nematode vector,Paratrichodorus allius, was investigated in greenhouse studies. ViruliferousP. allius multiplied on 24 out of 37 weed species tested, indicating they were suitable hosts of the vector. However, only 11 of these weeds were infected with TRV, as determined by ELISA. The nonhost status of a given weed species was not changed whether the viruliferous vector population originated from CRS problem fields in WA, OR, or ID. Several weeds served as hosts for the vector and virus including kochia, prickly lettuce, henbit, nightshade species (black, hairy, and cutleaf), common chickweed, and annual sowthistle. Virus-freeP. allius acquired TRV from the three nightshade species, volunteer potato grown from TRV-infected tubers, and prickly lettuce, and subsequently transmitted the virus to ‘Samsun NN’ tobacco indicator plants. Thus, some weeds may play a role in the epidemiology of CRS by perpetuating TRV and its vector in a problem field.

Tobacco rattle virus (TRV), the causal agent of corky ringspot disease of potato (CRS), was detected by reverse transcription-polymerase chain reaction (RT-PCR) in root tissues of field- and greenhouse-grown wheat (two) and corn (10) cultivars. TRV was also detected in some of the same samples by enzyme-linked immunosorbent assay (ELISA), but in a low frequency. Compared to ‘Samsun NN’ tobacco, wheat and corn appeared to be poorer hosts of TRV. However, these crops served as inoculum reservoirs of TRV. A nonviruliferous population of the nematodeParatrichodorus allius, the vector of TRV, acquired the virus from infected wheat and corn roots, and transmitted it to potato. ‘Russet Norkotah’ potatoes grown following TRV-infected corn and wheat, were blemished with arcs, concentric rings and diffuse browning, characteristic symptoms of CRS. The observations presented here suggest that wheat and corn as rotational crops will sustain TRV in a CRS problem field and the virus may cause severe damage on subsequent potato crops.

Soil samples from corky ringspot (CRS) problem fields of potato in the states of Washington and Oregon were collected and planted withNicotiana tabacum ‘Samsun NN’ tobacco to bait tobacco rattle virus (TRV) and to increaseParatrichodorus allius populations, the vector of TRV. Pathogenicity of three isolates of TRV was assessed on Russet Burbank and Russet Norkotah plants usingP. allius as the vector. The most severe CRS tuber symptoms were with TRV from Pasco, WA, followed by Umatilla, OR, and Mattawa, WA, indicating a distinct variation in virulence among virus isolates. The lowest number ofP. allius that transmitted TRV resulting in CRS symptoms on Russet Norkotah was three nematodes per 250 cm3 soil. When potato plants were exposed toP. allius at different times in their growth, tubers on older plants were more resistant than younger tubers to CRS. Severity of CRS tuber symptoms was correlated with age of potato plants in pot culture. Reproduction ofP. allius on potato roots did not influence the incidence and severity of tubers symptoms. Under similar conditions, Bintje, a variety known to be resistant to CRS, served as a host forP. allius, yet remained asymptomatic. The protocol employed in these studies is appropriate for testing potato germplasm for resistance to CRS.