Biocontrol of Tropical Soda Apple

A Highly Effective, Virus-Based Biological Control for Tropical Soda Apple UF# 10603 A Synopsis

Concept: To control tropical soda apple (Solanum viarum; TSA), a highly invasive noxious weed in the United States, by using an indigenous plant virus that causes an unusual, lethal, hypersensitive reaction in TSA.

In our search for a biological control agent for tropical soda apple we discovered that Tobacco mild green mosaic virus (TMGMV, previously called Tobacco mosaic virus, strain U2 [TMV U2]), causes a systemic, hypersensitive response in TSA plants that culminates in complete wilting of the plants. We have well characterized this host-virus interaction. In greenhouse and growth-chamber trials, 1- to12-month-old TSA plants are killed within a month (typically within 14 days) after manual inoculation with TMGMV. Younger plants are killed faster than older plants. The inoculated plants develop foliar local lesions, systemic necrosis of leaves and petioles, and systemic wilting in rapid succession within 7-14 days after inoculation. TSA is also susceptible to Tomato mosaic virus and Tobacco mosaic virus strain U1, but these tobamoviruses induce only mosaic and/or mottle symptoms. In repeated field trials, TMGMV has caused 83 to 97% mortality of TSA plants of different size and age categories.

Under controlled conditions in plant growth chambers, TMGMV caused 100% mortality of TSA plants at 18ºC and 32/22ºC cycles but plants at continuous 32ºC were asymptomatic. However, when these asymptomatic plants were transferred 17 days after inoculation and incubation at 32ºC to a greenhouse at 25-29ºC, they too developed systemic stem necrosis and eventually died. Thus, TMGMV has the unique capacity to induce systemic hypersensitive mortality in TSA.

Pathogen: As the name implies, TMGMV is regarded as a mild strain of Tobacco mosaic virus (TMV) that causes mild systemic foliar mosaic symptoms in susceptible plants such as Nicotiana glauca. This virus has been recognized as a distinct species of TMV since 1929. Over the years this virus has also been reported under the names of para-tobacco mosaic virus, green-tomato atypical mosaic virus, and TMV strain U5. Members of the Tobacco mosaic virus group (Tobamovirus group), including TMGMV, are mechanically transmitted viruses that are spread through physical contact between infected and healthy plants, through contaminated agricultural tools, and under unsanitary cultural practices. TMGMV is not known to be seed-borne. Members of the Tobamovirus group are not insect-transmitted in that they do not have a specific insect-vector relationship for transmission. However, some chewing and piercing insects may "mechanically" transmit the virus by first feeding on infected TSA plants and then moving to healthy plants. We are testing the possibility of such transmission in our laboratory.

Susceptible hosts: Based on published literature, TMGMV is known to infect 15 species of plants in four families. Studies done by F.W. Zettler and J. Nagel (University of Florida, Gainesville) established the frequent occurrence of TMGMV as a contaminant in nursery-grown plants of the Gesneriaceae family. We found the following solanaceous plants to be susceptible to TMGMV in our studies: Solanum macrocarpon and S. spinosissimum (developed mild mosaic and mottling symptoms); S. nigrum and S. rostratum (asymptomatically infected); and S. gilo, S. nodiflorum, S. americanum, S. sessilifolium, and S. pseudocapsicum (developed hypersensitive local lesions). Only Capsicum annuum (jalapeño and California Wonder) developed hypersensitive systemic necrosis comparable to that seen on TSA. Tomato and eggplant were immune to the virus.

We are now screening a large number of plants, representing both cultivated plants and weeds, to delineate the extent of host range of TMGMV. We will be able to identify potential nontarget hosts among cultivated plants, which may be exposed to the risk of infection by TMGMV, as well as identify possible additional biological control targets among weeds. A risk-benefit analysis of using TMGMV as a bioherbicide will be done.

Symptoms: In TSA plants, TMGMV causes hypersensitive necrotic flecks on inoculated leaves in about a week following inoculation. Then, within 2-3 weeks, the inoculated TSA plants begin to wilt and completely die. Occasionally, a few plants may produce suckers, but eventually the newly developing shoots have been observed to wilt. In susceptible tobacco (Nicotiana tabacum, cultivar nn Samsun) the virus causes systemic mild mosaic on the foliage. Resistant tobacco (cultivar NN Samsun) develops hypersensitive local lesions in response to TMGMV infection.

Environmental factors: The ability to use TMGMV as a control for TSA is not constrained by environmental factors. The virus is infective on TSA under a range of temperatures commonly found in spring, early summer, and fall in the South. Plant age is also not a limiting factor. Therefore, it is highly feasible to use this virus as a control for TSA.

Biocontrol efficacy: As stated above, in repeated trials conducted in the greenhouse and at two field locations, TMGMV provided more than 83% control of mature TSA plants. We are continuing to field testing the virus in Florida and our results are highly encouraging.

Cultural characteristics: TMGMV is extremely easy to produce in plants. Normally, the virus is multiplied in tobacco plants (nn Samsun) and the symptomatic leaves are harvested as they mature. The leaves are air-, vacuum-, or freeze-dried, ground into a fine powder, and stored over a desiccant (e.g., silica gel) at room temperature. Tobamovirus inoculum prepared and stored in this manner are known to remain highly infective for decades. According to one published source, longevity of TMGMV in vitro is 90 years! Thus, it should be fairly easy and inexpensive to produce and stockpile TMGMV inoculum year-round with a minimal capital outlay.

Research and development needs: Localized foliar necrosis is the most common type of virus-induced hypersensitive response in plants; hypersensitive plant death, as seen in the TSA-TMGMV system, is a rare phenomenon. Therefore, the genetic and physiological bases for the hypersensitive plant death in the TSA-TMGMV system beckon characterization. Specifically, we intend to identify and characterize the viral gene that elicits the wilting response in TSA. The anticipated outcome of this research is the identification of the TMGMV gene and gene product that trigger the systemic hypersensitive death at the cellular and tissue levels in TSA. This information may then be used to characterize the physiological mode(s) of action that control(s) the plant's death.

More immediately, the TMGMV-based control for TSA is ready to be developed further for use as a bioherbicide. We have contacted several cattle ranchers in Florida who have offered field sites for our proposed trials. We are also setting up a greenhouse-based pilot production facility to mass-produce and stockpile the virus inoculum. An operational production system will be set up and a cost analysis will be done. Virus yield, production costs, application cost per acre, and other relevant information will be developed.

We are looking for an industrial partner who is willing to invest in this technology to address the above-mentioned R&D aspects, obtain EPA registration, and quickly move this technology to the marketplace.