Compatibility of Plant Defense Elicitors with Aphid- and Nematode-Resistant Tomato Varieties in Integrated Pest Management
Fresh market tomato production is an important component of Southern agriculture, and is currently threatened by the disappearance of many commonly used pesticides from the market. A goal of our work is to identify effective alternative management strategies for two major pests of tomato: root-knot nematodes (Meloidogyne spp.) and potato aphids (Macrosiphum euphorbiae). This study utilizes standard bioassays to evaluate the efficacy of two classes of plant defense elicitors in controlling these pests. Benzothiadiazole (BTH), a synthetic analog of salicylic acid (SA), was used to induce SA-mediated plant defenses. This elicitor has previously been shown to induce resistance against certain pathogens, and may also be effective against aphids and nematodes. A second elicitor, jasmonic acid (JA), was used to elicit a different suite of defensive responses that have proven effective against certain insect pests and pathogens of tomato.
In addition to determining if BTH and JA are active against aphids and nematodes, we evaluated the compatibility of these elicitors with aphid- and nematode-resistant tomato varieties. Many commercial tomato varieties have been selected for resistance to root-knot nematodes and potato aphids. These resistant varieties are valuable and widely used, but they don’t provide complete immunity against aphids and nematodes. The proposed study will determine if plant defense elicitors could possibly enhance resistance in these tomato lines. The results from this study will contribute to the development of effective, sustainable, and environmentally sound pest management strategies for Southern tomato growers.
A major objective of this study is to determine if elicitors of SA- and JA-mediated plant defenses can protect tomato plants against aphid and root-knot nematode infestation. A second objective is to determine if the use of these elicitors is compatible with the use of tomato varieties that carry the aphid- and nematode-resistance gene Mi. Potentially SA and JA elicitors could enhance aphid and/or nematode resistance in these tomato varieties. Alternatively, they could interfere with Mi-mediated defenses, and render plants more susceptible to aphids or nematodes. Our experiments should determine whether or not these plant activators are appropriate for use on aphid- and nematode-resistant tomato varieties. To date we have found that both JA and SA defenses are compatible with Mi-mediated resistance and also impact aphid populations. We have found that JA defenses reduce avirulent nematode performance, but have yet to test the effect of SA defenses on nematode performance or JA defenses on virulent nematode performance.
1) The impact of an elicitor of SA-mediated plant defenses on aphid and nematode performance on tomato. In a factorial design, we compared aphid and nematode performance on A) an untreated tomato variety that lacks the Mi gene and is susceptible to aphids and nematodes (cv. Moneymaker); B) a near isogenic tomato line that carries Mi, and is resistant to aphids and nematodes (cv. Motelle); C) susceptible plants that have been pre-treated with an elicitor of SA-mediated defenses; and D) resistant plants that have been pre-treated with the SA-elicitor. The elicitor we have chosen to use for these studies is benzothiodiazole S methyl ester (BTH), an SA mimic that induces resistance to certain pathogens, and that is the active ingredient in the reduced risk pesticide Actigard (Syngenta Crop Protection). Two different potato aphid isolates (designated WU11 and WU12) that differ in their response to Mi were tested independently. One isolate, designated WU11, is heavily impacted by Mi-mediated resistance, while isolate WU12 is not reduced as greatly. The results from these bioassays indicated that both potato aphids are significantly reduced by activation of SA defenses. Furthermore, activation of SA defenses had an additive effect on resistance conferred by Mi, and helps reduce populations of aphid isolate WU12. The effect of BTH application on virulent and avirulent nematode isolates is currently being evaluated.
2) The impact of an elicitor of JA-mediated plant defenses on aphid and nematode performance on tomato. Application of JA-elicitors has been shown to induce resistance in tomato to caterpillars, and may also offer susceptible varieties some protection against aphids and nematodes (Thaler et al 2001; Zinov¡¦eva et al, 1998). A disadvantage of JA-elicitors, however, is that they suppress certain SA-mediated defense responses in tomato (Thaler et al., 1999). Because SA is thought to play a role in Mi-mediated resistance (Branch and Williamson, personal communication), JA elicitors could potentially interfere with this resistance. Therefore, we undertook to assess the compatibility of a JA elicitor with a resistant tomato variety, as well as to evaluate the benefits of this elicitor for use on susceptible varieties.
The effect of JA application was tested in the same manner as described for SA induction. The results from these bioassays indicated that both potato aphid isolates are reduced by activation of JA defenses. The effects of JA defenses did not appear to be as great as seen from BTH application. An additive effect over Mi mediated resistance was only observed from aphid isolate WU12. This may be a result of JA dependent defenses unable to further reduce aphid population already greatly impacted by Mi.
Several lifetable analyses and feeding assays have determined that JA defenses impacts aphid mortality. Daily individual fecundity was not impacted, however, total lifetime individual fecundity was impacted by JA application. Feeding studies revealed that JA does not impact feeding by aphids indicating that JA defenses are affected by having an anti-nutritive or toxic affect.
The effect of JA defenses was evaluated on avirulent root-knot nematode performance by counting total egg masses 6 weeks after JA application. The results from this study indicated that nematodes are reduced by activation of JA defenses. An additive effect was not seen over Mi mediated resistance due to the extremely low numbers of egg masses found on both JA treated and control resistant plants. The effect of JA application on virulent nematodes isolates is currently being evaluated.
3) The effects of temperature on the efficacy of our defense elicitors. If elicitors of either SA or JA-mediated defenses provide protection against nematodes (objectives 1 and 2), we will then determine if soil temperatures influence the impact of these elicitors. To this end we will measure nematode performance on treated and untreated plants at a series of three soil temperatures (27„aC, 30„aC, and 32„aC). One weakness of the nematode resistance provided by the Mi gene is that it is temperature-sensitive, and is not effective at soil temperatures above 30„aC (Dropkin, 1969). If defense elicitors can provide more temperature-stable protection, they could be of great value to Southern growers.
Presentations and Publications.
Results from JA bioassays were presented at the 2003 National Entomological Society Meeting in Cincinnati OH as a poster entitled “Compatibility of Induced Resistance and Mi-mediated Aphid Resistance in Tomato”.
A manuscript to be submitted to the Journal of Chemical Ecology is currently in preparation consisting of results from JA and BTH bioassays.
A manuscript to be submitted to Entomologia experimentalis et applicata is currently in preparation consisting of results from JA lifetable analyses and feeding assays.
Impacts and Contributions/Outcomes
The use of plants’ natural defenses is becoming increasing important to tomato production due to the continuing loss of broad-spectrum pesticides from the market. Commercially available products that induce JA and SA defenses are available for crop protection against certain insects and pathogens. The results from this study show that artificial induction of JA and SA significantly reduces both aphids and root-knot nematodes. This work is of practical value because susceptible tomato varieties play an important role in commercial tomato production. Although many tomato varieties carry the Mi resistance gene, susceptible tomato lines in some cases have desirable agronomic traits that are lacking in resistant varieties. Another benefit of susceptible tomato varieties is that, when grown in rotation with resistant lines, they may delay the emergence of resistance-breaking aphid and nematode strains. Therefore, it is important to identify effective means of controlling aphids and root-knot nematodes on susceptible varieties.
It is also important to evaluate potential interactions between induction of these defenses on resistant varieties commonly used in tomato production. The results from this study suggest the use of these defense elicitors are compatible with Mi, and can also be beneficially integrated for protection against nematodes and aphids. This information not only will help optimize aphid and nematode control, but also has practical value to growers who wish to use SA or JA elicitors to control other pests. For example, Actigard, a commercial preparation of BTH, is currently recommended for controlling bacterial speck on tomato (2002 product label, Syngenta Crop Protection). Growers who face problems with both bacterial speck and root-knot nematode therefore need to know if BTH will impact Mi-mediated resistance.
To date, we have shown both SA and JA defenses effect potato aphid populations and do not interact antagonistically with resistance conferred by Mi. Furthermore, SA and JA both enhance aphid protection offered by Mi. These results could have a positive contribution to the development of effective, sustainable, and environmentally sound pest management strategies for Southern tomato growers.