Selected novel approaches for the integrated pest management of Aphelenchoides fragariae in ornamental plants

Abstract

Leaf and bud nematodes ‘LBN’ (Aphelenchoides fragariae), are microscopic widespread pests of the ornamental industry causing distortion and angular-shaped lesions / blotches on leaves of woody, perennial and herbaceous plants worldwide. They cause an annual loss of estimated millions of dollars on affected plants. These pests spread and infest aerial plant parts by various means including leaf touching from infested plants to healthy plants; movement of LBN in water films during rainfall, misting or irrigation from infested to clean plants; nematode presence in infested leaf debris found on the surface of soil / planting media, sand beds or ground / floor cover matting. These are just some of the infestation routes by which nematodes can be spread in the field. Growers can inadvertently transmit LBN via cut materials taken from infested mother stock, especially when the mother plants are asymptomatic. Symptoms become visible as the new plant grows and nematode numbers build up. Therefore, cleaning and sterilisation of implements / pots are important to reduce the chance of nematode spread to healthy plants. LBN can also be spread from the soil where they can overwinter as juveniles and adults (not as eggs) for some months, and sustenance can be maintained in the soil by feeding on saprophytic fungi in the absence of host plants. In addition, they can overwinter in the plant parts such as buds, rhizomes and bulbs, but not in the root. After sexual reproduction takes place, the life cycle from egg to adult is generally completed in 10-11 days at 18oC. The nematode exhibits both ecto- and endoparasitic lifestyles. LBN moves up externally of the plant during spring to invade the new leaves through natural stomata or wounds. Symptoms include deformation of buds, leaves and flowers causing brown to black, or chlorotic, vein-delineated angular lesions that can become necrotic. If buds or young leaves are infested, they may not develop properly and may become deformed, and this would render such plants unmarketable. Since ornamental plants are sold for their aesthetic value, infestation leading to visual symptoms should be prevented in order to avoid economic loss. Management of A. fragariae has become challenging because of the revocation and subsequent loss of systemic pesticides, lack of approved bio-pesticides products, mis-diagnosis of symptoms, continuous movements of asymptomatic plants and increased production of these vegetatively propagated plants. In the UK, A. fragariae and A. ritzemabosi are the two main LBN of economic importance. The last approved nematicide in the UK was Vydate 10G (oxamyl) with an Extension of Authorisation for Minor Use (EAMU) for use on protected ornamental plants, which expired at the end of December 2017. Therefore, there is a need for Vydate 10G’s replacement to be identified and evaluated for the management of LBN, along with an improvement on the cultural control methods used, as an important component of integrated management of these pests. To develop new approaches for the management of LBN using A. fragariae as a model species in ornamental plants, this project evaluated individually, and in combination, the efficacy of currently approved pesticides including Movento (spirotetramat) and Dynamec (abamectin), elicitor treatments ‘acibenzolar-S-methyl’ (ASM), known to induce resistance against pest in plants, and some bio-pesticide products derived from plant extracts such as azadirachtin, isothiocyanates and garlic extract. Experiments were conducted in bioassays for contact mortality to A. fragariae, inoculation methods were developed and adapted for nematode screening purposes, and used during glasshouse and commercial nurseries to investigate efficacy of these products, and subsequent foliar application of curative and preventative approaches on (naturally and artificially) infested plants, with these products in glasshouse and nursery conditions. Results showed that isothiocyanates, garlic and abamectin had >75% contact mortality to A. fragariae in water bioassays. The elicitor ASM significantly reduced the population of A. fragariae by up to 60% compared with untreated Control in a curative approach after a 3x foliar application programme in ornamental plants. A curative approach method on 9 naturally infested plants (Gunnera manicata, Anemone hupehensis, Cistus corbariensis, Buddleja davidii, Bergenia cordifolia, Astrantia major, Brunnera macrophylla, Astilboides tabularis, Dryopteris filix-mas) indicated that all the treatments led to a >60% reduction of nematode population over the untreated Control (ROC%). The highest reduction was obtained with a combination of ASM + spirotetramat on most evaluated plants. A preventative approach with the use of azadirachtin, abamectin, spirotetramat and ASM on artificially inoculated plants as single product programmes, and in combination with ASM led to low (73-609) mean nematode populations per 1g leaf, compared to the Control populations of 2454-5005 per 1g of leaf eight weeks after nematode inoculation on B. davidii and A. hupehensis plants. The lowest mean population (73) was obtained from the spirotetramat + ASM programme. As a preventative approach, ASM applied alone in a spray programme on Anemone hupehensis inoculated with 200 nematodes / leaf, had a mean nematode population of 255 compared with 1757 nematodes from untreated control 8 weeks after inoculation. A glasshouse test was conducted on LBN infested soil media with 6 products to evaluate activity in preventing plant invasion from the soil. Oxamyl and treatments such as Bacillus thuringiensis (biological), fluopyram (fungicide), garlic extract, isothiocyanates & capsicum and Bacillus firmus (biological) limited nematode movement from infested soil media to the plant with reduced nematode multiplication within the leaf. The nematode symptom visual rating assessment guide developed in this thesis identified a correlation between nematode symptom severity (leaf lesions) and nematode population within the affected leaf. This guide will also help growers to improve on identification of LBN symptoms on leaves at the early stage. It will be useful in making decision for immediate action to prevent further spread of infestation by treating symptomatic plants with less than 15% leaf area damage (LAD) or dispose of plants with over 15% LAD in which such infested leaves may not likely respond to any treatment applied thereafter. Results from field studies have demonstrated the potential of several novel products to manage LBN, and the potential for ASM combined with azadirachtin, abamectin and spirotetramat. As a preventative approach, a foliar spray programme of ASM should be considered to prime plants ahead of LBN symptoms, while a combination of ASM + spirotetramat or abamectin is suggested for a curative approach on plant exhibiting symptoms. Both spirotetramat and abamectin are currently registered in the UK for insect control in ornamental crop production, and the elicitor ASM approved for use on protected chrysanthemum. It is important to note that treatment will be most effective at the first sign of nematode symptoms when plants are actively growing. Considering various potential control methods of LBN, the combination of elicitor with insecticides offered the best control methods in this study compared to results from pesticide or elicitor as a stand-alone treatment. The effectiveness of combined elicitor + insecticides programmes is likeley to have been due to the elicitor increasing plant resistance against further nematode multiplication, while the insecticides, known to reduce inoculum levels, have acted on the nematode population, either by systemic or contact action, thereby leading to a significant reduction of nematode levels compared with sole candidate treatments. However, the application of cultural control methods and a high level of hygiene, when incorporated with the above treatments in a practical IPM approach, will enhance LBN management on ornamental plants.