Location: Vegetable Crops Research
Project Number: 5090-21220-006-040-S
Project Type: Non-Assistance Cooperative Agreement
Start Date: Aug 1, 2023
End Date: Jul 31, 2025
Objective:
Insecticides and miticides are used by potato producers across the US to control many of the major insect and mite pests. Colorado potato beetle (Leptinotarsa decemlineata, CPB) is one of the key insect pests in midwestern and eastern potato production regions and more recently has emerged as a problem in western production. In general, agronomists and plant protection practitioners deal with CPB by applying insecticides when specific stages of the population have been reached. However, insects are typically not the only pest that agronomists/scouts encounter, especially given that plant disease management is a season-long challenge in potato grown in high humidity environments. This often leads to a complex array of insect and disease threats occurring in the field simultaneously. As a result, managers rely on the use of a combination of insecticides and fungicides to manage these pest complexes. Moreover, many farm operations are adding foliar-applied nutritional supplements to the developing crop to optimize tuber set, tuber quality and overall yield and recovery at the end of the season.
A current trend in the specialty crop industries is the loss of older, conventional pesticides including insecticides and fungicides that kill a broad range of pests and pathogens. This loss has led to the registration of pesticides with a narrow range of pest or disease activity or selectivity, and these are often referred to as biorational pesticides. However, in order to continually manage the diversity of insect and disease threats, field managers are necessarily needing to mix together or "tank mix" several biorational pesticides to broaden the spectrum of activity of the application. The primary benefit of tank mixing is a reduction in the number of applications, thereby decreasing labor and input costs and gaining greater efficiency in large potato operations that have many acres to treat. In some rare instances, tank mixing two pesticides may result in enhanced mortality of insects or diseases than if either pesticide were used separately. Despite the initial benefits of these types of rare pesticide mixtures, most often what results are problems that arise when two or more pesticides are mixed together, and each are expected to perform optimally. These include increasing the probability of insect or disease resistance to multiple pesticides, potential plant injury (phytotoxicity) and pesticide incompatibility. An even greater concern is antagonism that can occur among tank mix partners. This occurs when the mixing of two or more pesticides results in lower pest or pathogen mortality than if the pesticides were applied separately.
The purpose of this study is to, i) determine if mixtures of biorational pesticides and nutritional supplements that are labeled for and used to control CPB and early blight (Alternaria solani) in potato results in reduced efficacy (antagonism) using field-based bioassays, and ii) evaluate for potential antagonism in the control of select diseases including late blight (Phytophthora infestans) and A. solani using detached-leaf bioassays under controlled, laboratory-based conditions.
Approach:
Objective I. Field based bioassays measuring antagonism. Four insecticides commonly used to manage CPB will be screened in a field based experiment to determine if two- or three-way combinations with fungicides and foliar nutrients will result in any antagonistic effects in the control of CPB. Similarly, fungicides used as base protectants and as systemic rescue treatments will be evaluated in two- and three-way interactions with insecticide and foliar nutrient mixes to observe any antagonism in terms of A. solani occurrence and associated disease development. Briefly, a randomized complete block design (RCBD) will be used with six replications per treatment and will include the following 4 insecticides treatments: spinetoram, abamectin, abamectin and dsRNA applied separately and in all possible mixtures. Similarly, a RCBD with 6 experimental replicates will also include the following 4 fungicides: chlorothalonil, mancozeb, boscalid, and difenoconazole, again applied individually and within the mixture of insecticides and foliar nutrients. Foliar-applied nutritional components will include the following five micronutrient mixes: BioGrow 3-12-0, BioForge, NUE Mag, Max Set 28 and SetMore Base. All experimental plots will consist of 4 rows measuring 30’ in length with rows spaced on 3 ft. centers. Due to the large number of treatments in this trial, the field will be separated into blocks of 12 treatments/block where each will contain an untreated control. Potato will be machine-planted and placed on approximately 12 in. row spacing. In addition to the treatments proposed, plots will be maintained according to standard commercial practices including herbicide, and irrigation management. To assess pesticide compatibility in field-based experiments, a jar test will be conducted for each pesticide and mixture combination and visually evaluated for layering, precipitate formation and settling 0, 2, 5 and 20 minutes after mixing.
Objective II. Detached leaf bioassays to investigate the performance of pesticide mixtures. Cut whole leaves from mature S. tuberosum cv. Russet Burbank will be used to perform detached leaf assays (DLA) to test for infection and lesion growth following inoculation with P. infestans. Mature, fully expanded whole leaves will be cut to a uniform length (6-8 inches) and dip inoculated with a set of pesticide mixtures similar to those described previously in Obj. 1. Briefly, replicate sets of whole detached leaves will be dip-inoculated with field relevant rates of the following fungicides: chlorothalonil, mancozeb, oxathiapiprolin, and mandipropamid. Infection and lesion size of P. infestans and A. solani inoculations on whole leaves pre- treated with these fungicides individually will be compared with the full mixtures containing premixes with insecticides and foliar nutrients. First symptoms can be observed 3 days after inoculation as black/brown lesions, together with sporulation that can be observed around water-soaked area at the point of pathogen inoculation. Leaves can be visually scored for the appearance of symptoms using ImageJ software 5 days after inoculation.
