Implications of weed composition and thrips species for the epidemiology of tomato spotted wilt in chrysanthemum (Dendranthema grandiflora)

The effects of 21 weeds acting as hosts of tomato spotted wilt virus (TSWV), temperature, thrips population and diversity on disease progress in chrysanthemum cv. Polaris were studied. Under greenhouse conditions, only Taraxacum officinale, Bidens sp., Resedaluteola and Mirabilis jalapa were hosts for TSWV. Of 38 weeds species in the area surrounding a chrysanthemum field, Tithonia tubaeformis and R. luteola had the highest populations of adult and immature thrips. These weeds, as well as M. jalapa, had an extensive seasonal distribution and may play a key role in the disease progress. Seventeen thrips species belonging to the genera Bravothrips, Thrips and Frankliniella were identified on weed flowers, with Frankliniella occidentalis (FOC) representing 9.5% of all thrips identified. Of 123 thrips collected from chrysanthemum inflorescences, 9.75% were FOC, and only 2.5% of them transmitted TSWV. Of all the thrips species collected from chrysanthemum flowers in the field, only FOC was capable of transmitting TSWV. On 120 experimental plots established at two sites, with three transplanting dates (June, July and August), it was estimated that 1.25% of the chrysanthemum cuttings were already infected with TSWV when transplanted. Secondary spread, vectored by FOC, occurred only for the earliest transplanting date and resulted in a further 2.36% disease incidence.