An Environmentally-Friendly Alternative for Control of the Citrus Nematode in Arizona

The citrus nematode, Tylenchulus semipenetrans, has been reported to affect 90% of Arizona’s citrus. At present few options exist for control of this pest. Most chemical nematicides used in the past are no longer available. This situation urges the search for new control alternatives. Biocontrol agents such as entomopathogenic nematodes (EPN) are one potential option. EPN have been shown to control some plant–parasitic nematode species. In this project we propose to assess EPN for control of T. semipenetrans. If proven effective, EPN will provide an alternative to chemical-control for implemention in Arizona and other citrus-producing regions in the country.

Introduction

The citrus nematode, Tylenchulus semipenetrans (Nematoda: Tylenchulidae), is one of the most debilitating citrus pests worldwide (Duncan and Cohn, 1990; Verdejo-Lucas and Kaplan, 2002). This nematode is an obligate parasite that reproduces only on the living roots of host plants. The nematode female becomes semi-endoparasites and sedentary by burrowing its anterior end deep inside the root cortex while the posterior end remains outside in the soil. It establishes feeding sites within the root cortex composed of nurse cells that surround the female nematode head. The posterior portion of the adult female protrudes from the root and is surrounded by a gelatinous matrix in which eggs are deposited (Cohn, 1965).
In Arizona T. semipenetrans was first discovered in 1926 and, since then, citrus growers have faced a continual battle with this citrus pest. Ninety percent of the citrus in the state has been reported to be affected by this nematode (Olsen et al., 2000). Although this nematode does not kill the trees, it significantly reduces tree vigor, growth and grove productivity (Duncan and Cohn, 1990). For this reason, the disease is often referred to as a “slow-decline.” Above-ground symptoms can include leaf yellowing, sparse foliage, small and non-uniform fruit and defoliated upper branches. Yield losses caused by T. semipenetrans are estimated at about 10% worldwide or $77 million per year (Verdejo-Lucas and Kaplan, 2002). At present very few alternatives are available for control of this nematode. Most chemical nematicides previously considered for control of this nematode (i.e. aldicarb, fenamiphos, DBCP) have been or soon will be removed from the market due to their known toxicity and detrimental health effects to humans, wildlife and soil and ground water contamination (McClure and Schmitt, 1996; Anonymous, 2002). This situation and the increasing awareness of environmental and human health concerns associated with chemical nematicides urges the search for new alternatives of environmentally friendly products for management of this nematode. In this respect, one of the choices for substitution of chemical nematicides is the consideration of biological control agents such as entomopathogenic nematodes (hereafter referred as to EPN). EPN provide an environmentally safe and economically reasonable alternative for a wide range of arthropod pests and plant-pathogens (Tanada and Kaya, 1993). With respect to plant-parasitic nematodes, several greenhouse experiments and field trials have shown the ability of EPN to reduce plant–parasitic nematode penetration and egg production (Lewis et al., 2001; Jagdale et al., 2002; Pérez and Lewis, 2004). EPN are already available as commercial products and can directly be used for nematode control thus reducing cost and time of product development. Preliminary laboratory tests conducted at U of A with Steinernema riobrave show good promise of this EPN to control citrus nematode. Two application types were considered: nematode aqueous suspension and EPN-infected cadavers. Our results indicate that S. riobrave-infested cadavers in the soil significantly reduce egg production and penetration in rough lemon seedlings (Gress & Stock, 2005. Abstract- 10th European IOBC Meeting)

Over the past decades, the prevailing use of chemical pesticides has generated several problems including insecticide resistance, outbreaks of secondary pests, safety risks for humans and domestic animals, contamination of ground water, decrease in biodiversity among other environmental concerns (Lacey et al., 2001). These problems and sustainability programs based mainly on conventional insecticides have stimulated increased interest in the development and implementation of cost-effective, environmentally safe alternatives to chemical pesticides for arthropod and plant-pathogens control. Sustainable IPM in the 21st century will rely increasingly on alternative strategies for pest management that are environmentally friendly and reduce the amount of human contact to chemical pesticides. One of the most promising choices to help minimize usage of chemical pesticides is the implementation of EPN. Many species are currently employed as biological control agents of insect pests and plant-parasitic nematodes in row and glasshouse crops, orchards, ornamentals, range, turf and lawn, store products and forestry (Lacey et al., 2001). With EPN, we have the opportunity to develop and implement technology that will significantly reduce the transmission of disease, protect biodiversity, enhance water quality, preserve the environment and improve food safety and affordability. This aspect is of crucial need in desert ecosystems like the one in southwestern U.S.

In this project we assessed EPN as an alternative tool for control of the citrus nematode, T. semipenetrans. Use of EPN as a substitute for chemical pesticides for control of agricultural pests is not a novel idea, but no EPN have yet been tested against this specific plant-parasitic nematode. In order to effectively implement EPN for the control of this plant-parasitic nematode, some basic research needs to be established. In this respect, data generated from this study will assist in the subsequent development and application of EPN as an alternative to citrus pest control practices. If proven effective, the consideration of EPN for control of this plant-parasite will provide an environmentally safe alternative to traditional chemical control that could be implemented not only in Arizona, but in other citrus producing regions in the country. This aspect intimately relates to Western SARE goals 1 and 5, which promote profitable sustainable farming methods to help maintain and enhance the quality of the soil, and conserve natural resources and wildlife. In addition to this, the effective control of this citrus pathogen will eventually encourage growers to continue growing citrus in Arizona, therefore increasing crop diversification in the Southwest. This aspect relates to Western SARE goals 2 and 4.

Moreover, the proposed research also correlates to Western SARE goal 3, which advocates the adoption of methods and agricultural practices that reduce potential risks to human health and the environment caused by pests themselves or by the use of pest management practices. Furthermore, the development an environmentally safe and effective biocontrol agent (i.e. EPN) could greatly improve crop and yield quality, therefore enhancing the quality of life of citrus growers in this region. This aspect is addresses Western SARE goal 2, which promotes the enhancement of quality of life of farmers and ranchers and ensure the viability of rural communities by increasing income and employment in agricultural and rural communities.