Vascular Puncture of Maize Kernels for the Mechanical Transmission of Maize White Line Mosaic Virus and Other Viruses of Maize

Abstract

Maize white line mosaic virus (MWLMV) was efficiently transmitted (>90%) by a vascular puncture method with insect pins to maize kernels without causing teratogenic or lethal injuries to the developing seedlings. The pins, soldered to a 10-gauge copper wire, were used to puncture vascular tissues of maize kernels. The copper wire and pins were mounted in an engraving tool for machine-assisted inoculation. Effects of preinoculation soaking of kernels in water; postinoculation moisture regimes; the size, number, and configuration of pins; and the site and frequency of inoculation on rates of transmission were evaluated. Transmission rates of MWLMV were highest (average 32%) when the preinoculation soaking of kernels was done at 4 C. In contrast, transmission rates were significantly reduced when the preinoculation soaking of kernels was done at 30 C. The average rates of transmission to inoculated kernels incubated at 30 C on paper towels moistened with either 50 or 150 ml of water in a 2-L Pyrex dish for 24 h or directly planted into soil were 42, 20, and 11% (P > 0.003; LSD = 14.5%), respectively. Inoculations near the side of the embryo averaged 58% transmission whereas inoculations near the tip of the plumule averaged 33% transmission (P > 0.0001). Subjecting kernels to one, two, or three preinoculation soak (24-h) and dry (20- to 24-h) cycles or a 4- or 20-h preinoculation soak period resulted in transmission rates of 95, 90, 87, 80, and 14% (P > 0.0001), respectively. The optimum conditions for MWLMV transmission were 1) the use of a machine-assisted inoculator with minuten pins positioned like the tines of a fork, 2) preinoculation soaking of the kernels at 21 C for 4 h, 3) inoculation of kernels near the side of an embryo, and 4) incubation of kernels at 30 C postinoculation for 24 h on paper towels moistened with 50 ml of water. The ranges of transmission rates with this protocol for the following maize viruses were maize chlorotic dwarf waikavirus, 1-34%; maize dwarf mosaic potyvirus (strain A), 41-82%; maize mosaic rhabdovirus, 1-19%; maize rayado fino marafivirus, 1-25%; maize rough dwarf fijivirus (maize Rio Cuarto disease), 1 %; maize streak geminivirus, 1-5%; maize subtle mosaic virus, 12-48%; and wheat streak mosaic potyvirus, 3-55%