Spotted wilt virus disease caused by
Spotted wilt disease caused by TSWV is a serious disease in peanut in the United States (
TSWV is transmitted by at least seven thrips species belonging to the insect Order
Controlling the vector chemically seems to have no or little impact in reducing disease incidence, and other alternative means of managing this disease are needed (
Recently, several breeding lines have shown greater field resistance to TSWV in the eastern U. S. (
During the 2001, 2002, and 2003 growing seasons, peanut cultivars Tamrun-98, Tamrun-96, Southwest Runner, Georgia Green, Tamrun OL-01, Georgia Hi Oleic, Tamrun OL-02 and Okrun were planted in field plots near Pearsall, TX, where TSWV disease is endemic. In all growing seasons, field plots consisted of eight cultivars (treatments) arranged in a randomized complete block design with four replications. In each replicate, cultivars were planted in two rows 6 m long with row spacing of 0.91 m. Number of plants per plot was between 120–130 which was lower than commercial fields to promote higher incidence of spotted wilt disease (
Area under disease progress curve (AUDPC) was calculated for each plot as described by
All data were subjected to statistical analysis of variance (
The greenhouse experiment was designed as an 8 × 3 × 2 factorial arrangement of treatments (8 genotypes × 3 inoculation times × 2 inoculation treatments) in a randomized complete block design (RCBD) with four replications. Seeds were germinated in trays lined with water-saturated- filter paper (Whatman #1). Each germinating seed was planted in a polyvinyl chloride (PVC) tube (10 cm inside diameter and 76 cm in length) filled with fritted clay as described by
In the 2001 growing season, TSWV-like symptoms were first observed at 30 days post-planting (DPP) in all peanut genotypes tested. Leaves from symptomatic plants were collected and tested for the presence of TSWV by ELISA, where values ranged from 0.28 to 1.33. TSWV disease incidence in 2001, in all peanut cultivars increased with time (
Reaction of peanut cultivars to Tomato Spotted Wilt Virus in field plots at Pearsall, TX, in 2001.
Reaction of peanut cultivars to Tomato Spotted Wilt Virus in field plots at Pearsall, TX, in 2002.
Reaction of peanut cultivars to Tomato Spotted Wilt Virus in field plots at Pearsall, TX, in 2003.
During the 2001 growing season, AUDPC values of spotted wilt of Georgia Hi Oleic, Georgia Green, and Tamrun-96 were 162, 206 and 212, respectively, which were lower than those of Okrun, Tamrun-98, and Southwest Runner (310, 341, and 372, respectively). Similarly, in the 2003 growing season, the AUDPC of Georgia Hi Oleic, Georgia Green, and Tamrun-96 were 120, 125 and 187, respectively, which were lower than those of Southwest Runner, Tamrun-98, and Okrun (208, 215 and 230, respectively). During the three growing seasons, disease incidence of TSWV varied between the peanut cultivars tested. Georgia Green, Georgia Hi Oleic, and Tamrun-96 cultivars exhibited greater resistance to TSWV compared to the other cultivars tested.
The resistance of Georgia Green to TSWV observed in our plots in 2001, 2002, and 2003 corroborated with the partial resistance reported by other researchers (
More than 90% of TSWV-inoculated peanut plants grown in the greenhouse, of all peanut cultivars expressed foliar symptoms. Plants inoculated with buffer only (Controls) did not exhibit TSWV symptoms. Expression of TSWV on inoculated peanut cultivars is shown in
Disease severity index (DSI) 1 of mechanically inoculated peanut cultivars with Tomato Spotted Wilt Virus under greenhouse conditions.
It was reported that Georgia Green did not express the observed level of field resistance to TSWV following mechanical inoculation. The resistance of Georgia Green in the field might be effective only against thrips-borne inoculum, and might have broken down against the relatively high dosage applied in a mechanical inoculation (
Significant differences were observed in all inoculated plants compared with non-inoculated controls at all three mechanical inoculation times under greenhouse conditions. Readings of ELISA in inoculated planted showed variations (
ELISA values* of mechanically-inoculated peanut cultivars with Tomato Spotted Wilt Virus under greenhouse conditions.
Root lengths of inoculated plants and non-inoculated controls are presented in
Root length (cm) of mechanically-inoculated peanut cultivars with Tomato Spotted Wilt Virus under greenhouse conditions.
When non-inoculated controls and inoculated plants were compared for root volume, significant differences were revealed at all three inoculation times (
Root volume (cm3) of mechanically-inoculated peanut cultivars with Tomato Spotted Wilt Virus under greenhouse conditions.
When comparing non-inoculated controls and inoculated plants at 5 DPP, plant height of Tamrun OL 01 was reduced the least (3%), which was significant (P < 0.05) from Tamrun-98 only (
Plant height (cm) of mechanically-inoculated peanut cultivars with Tomato Spotted Wilt Virus under greenhouse conditions.
Fresh weights of peanut cultivars of TSWV-mechanically-inoculated and non-inoculated controls were presented in
Plant fresh weight (g) of mechanically-inoculated peanut cultivars with Tomato Spotted Wilt Virus under greenhouse conditions.
When peanut cultivars were inoculated at 5 DPP, significant differences in relative water content (RWC) were observed in Georgia Green and Southwest Runner compared to non-inoculated plants (
Relative water content (RWC %) of mechanically-inoculated peanut cultivars with Tomato Spotted Wilt Virus under greenhouse conditions.
Positive correlations were obtained between DSI and ELISA values among most inoculated cultivars (
Correlation coefficient between Tomato Spotted Wilt Virus disease severity index and plant root length and root volume, relative water content (RWC) and ELISA readings in peanut cultivars grown under greenhouse conditions.
Under controlled conditions,
Screening for plant resistance to viral agents via mechanical inoculation can provide useful information on genetic variations within the peanut germplasm. The identification of such genetic variations is important in breeding efforts to develop genotypes with resistance to TSWV. Mechanical inoculation, to screen for resistance under controlled conditions can be performed throughout the year, regardless of seasonal variations. However, it has the disadvantage of excluding the role of the vector in determining the observed reaction.
Screening peanut genotypes under field conditions has the advantage of identifying inherent viral resistance, and possibly vector preference and/or genotypes resistant to the thrips that can also provide indirect strategies for disease management to TSWV. However, the disadvantages of field screening include; limiting the evaluation to the growing season, variation of TSWV pressure from one location to another and yearly variation. In addition, the field environment with its complex biotic and abiotic agents may interfere either positively or negatively with identification of resistance. Our data of the field and greenhouse evaluations indicate the potential usefulness of the peanut cultivars Georgia Green, Tamrun-96, and Georgia Hi Oleic for managing TSWV in peanut production in Southwestern United States.
1Mohammed A. AL-Saleh, Former Graduate Student
2*Hassan A. Melouk, Plant Pathologist and Professor, USDA-ARS, Plant Science Research Laboratory, Department of Entomology and Plant Pathology, Oklahoma State University, Stillwater, OK 74078.
3Phillip Mulder, Professor of Entomology, Department of Entomology and Plant Pathology, Oklahoma State University, Stillwater, OK 74078.