Effect of nucleopolyhedrovirus concentration in soil on viral transport to cotton (Gossypium hirsutum L.) plants

Soil-to-plant abiotic transport of a recombinant nucleopolyhedrovirus (HzSNPV.LqhIT2) was studied to quantify the proportion of different concentrations of soil virus transported to specific parts of cotton plants under controlled greenhouse conditions; these results were related to transport in the field where wind, rain, and soil type were not controlled. Under conducive precipitation conditions in the greenhouse, the estimated number of viral occlusion bodies (OB) transported ranged from 7 OB (to the top third of the plant, 40-60 cm above the soil, at the low virus concentration, 250 OB/g soil) to 629 OB (to the bottom third of the plant, 0-20 cm, at the high virus concentration, 12,500 OB/g soil). Under conducive wind conditions in the greenhouse, the estimated number of OB transported ranged from 8 OB (to the top third of the plant at the low concentration) to 94 OB (to the bottom third of the plant at the high concentration). The overall proportion of OB transported from soil to plant was greatest, ranging from 2.1-6.2 x 10-⁶, from the lowest soil concentration to the lowest 40 cm of the plant. Only 5 x 10-⁸ of the soil OB were transported from the high-concentration soil to a height of 40-60 cm on the plants. In the field experiment, the estimated number of OB on each cotton plant depended on the concentration of OB in soil in June and July, but this effect was no longer significant in August. There were significantly more OB on the lower third of plants than on the top third in July, but not in June or August. Significantly more OB were detected on cotton leaves than on buds or squares in July, and there were more OB on leaves than on buds, squares, bracts, or bolls in August. The amount of HzSNPV.LqhIT2 naturally transported from soil to cotton plants was sufficient to infect 6-11% (low to high soil concentration) of first instar Heliothis virescens (Fabricius) (Lepidoptera: Noctuidae) in June, 2-6% in July, and 1-3% in August. These results fill gaps in understanding NPV epizootiology that are important to biological control and risk assessment.