Application of Plant Defense Elicitors Fails to Enhance Herbivore Resistance or Mitigate Phytoplasma Infection in Cranberries

Abstract

Synthetic elicitors of the salicylic acid (SA) and jasmonic acid (JA) plant defense pathways can be used to increase crop protection against herbivores and pathogens. In this study, we tested the hypothesis that elicitors of plant defenses interact with pathogen infection to influence crop resistance against vector and nonvector herbivores. To do so, we employed a trophic system comprising of cranberries (Vaccinium macrocarpon), the phytoplasma that causes false blossom disease, and two herbivores—the blunt-nosed leafhopper (Limotettix vaccinii), the vector of false blossom disease, and the nonvector gypsy moth (Lymantria dispar). We tested four commercial elicitors, including three that activate mainly SA-related plant defenses (Actigard, LifeGard, and Regalia) and one activator of JA-related defenses (Blush). A greenhouse experiment in which phytoplasma-infected and uninfected plants received repeated exposure to elicitors revealed that both phytoplasma infection and elicitor treatment individually improved L. vaccinii and L. dispar mass compared to uninfected, untreated controls; however, SA-based elicitor treatments reduced L. vaccinii mass on infected plants. Regalia also improved L. vaccinii survival. Phytoplasma infection reduced plant size and mass, increased levels of nitrogen (N) and SA, and lowered carbon/nitrogen (C/N) ratios compared to uninfected plants, irrespective of elicitor treatment. Although none of our elicitor treatments influenced transcript levels of a phytoplasma-specific marker gene, all of them increased N and reduced C/N levels; the three SA activators also reduced JA levels. Taken together, our findings reveal positive effects of both phytoplasma infection and elicitor treatment on the performance of L. vaccinii and L. dispar in cranberries, likely via enhancement of plant nutrition and changes in phytohormone profiles, specifically increases in SA levels and corresponding decreases in levels of JA. Thus, we found no evidence that the tested elicitors of plant defenses increase resistance to insect herbivores or reduce disease incidence in cranberries.

Advances in cranberry insect pest management: A literature synthesis

“…In some cases, plant diseases, such as cranberry false blossom, can interfere with the levels of nutrients and defenses in cranberry plants (Pradit et al, 2019;Pradit et al, 2020;Rodriguez-Saona et al, 2021), which can affect L. vaccinii development (Pradit et al, 2019), as well as benefit non-vector pests, like S. sulfureana, C. parallela, and L. dispar (Pradit et al, 2020). Moreover, commercial elicitors of plant defenses, such as activators of the salicylic acid (SA) and jasmonic acid (JA) defense pathways, failed to increase cranberry resistance against insect herbivores as well as the disease-causing phytoplasma (Rodriguez-Saona et al, 2021). However, the application of elicitors of the JA defense pathway can lead to higher volatile production that can potentially attract natural enemies of insect pests (de Lange et al, 2019b;Rodriguez-Saona et al, 2021) or repel pests, such as S. sulfureana (Rodriguez-Saona et al, 2013).…”

“…Moreover, commercial elicitors of plant defenses, such as activators of the salicylic acid (SA) and jasmonic acid (JA) defense pathways, failed to increase cranberry resistance against insect herbivores as well as the disease-causing phytoplasma (Rodriguez-Saona et al, 2021). However, the application of elicitors of the JA defense pathway can lead to higher volatile production that can potentially attract natural enemies of insect pests (de Lange et al, 2019b;Rodriguez-Saona et al, 2021) or repel pests, such as S. sulfureana (Rodriguez-Saona et al, 2013).…”

“…As farmers adopted new reduced-risk insecticides against lepidoptera pests in the past two decades, such as IGRs, spinosyns, and diamides, populations of pests that are not affected by these insecticides, like L. vaccinii, have been on the rise (de Lange and Rodriguez-Saona, 2015a). This, in conjunction with a shift in cranberry breeding from insect resistance to fruit quality (Vorsa and Zalapa, 2020), has resulted in the increased incidence of false blossom disease in New Jersey, namely because the only effective control of the disease is to target the vector (Rodriguez-Saona et al, 2021). This pest resurgence has been exacerbated by the recent banning of the organophosphate chlorpyrifos (Lorsban ® ) that was widely used to control L. vaccinii.…”