Evaluation of Biological Control Agents and Conventional Products for Post-Harvest Application on Potato (Solanum tuberosum L.) to Manage Leak

Biological control agents applied post-harvest may provide an effective way to manage leak of potatoes by competing with Pythium ultimum. The objective of this paper was to test the efficacy of various biological control agents and conventional post-harvest fungicides to manage leak. Two studies were performed with a Pseudomonas fluorescens triculture including desiccation tolerant variants of strains S11P12, P22Y05, and S22T04 in three formulations (a “fresh” triculture, dried on Kenite 700, or dried on Attapulgite clay) applied at a rate of 3.5 mL kg⁻¹. A third study contained treatments of the P. fluorescens “fresh” triculture, triculture dried on Kenite, triculture dried on Kenite blended with a fungicide containing a three-way mixture of azoxystrobin, fludioxonil, and difenoconazole (Azo+Flu+Dfz, 0.033 mL kg⁻¹), and triculture dried on Kenite rehydrated with nutrient broth Medium 1 blended with the same fungicide. Other treatments included Pseudomonas syringae (3.5 mL kg⁻¹) in Studies 1 and 2, while hydrogen peroxide and peroxyacetic acid (0.042 mL kg⁻¹), phosphorous acid (4.2 mL kg⁻¹), and Azo+Flu+Dfz (0.033 mL kg⁻¹) were used in all three studies. The three studies were each repeated twice using unwashed tubers (cv. ‘Russet Burbank’) that were wounded and inoculated with P. ultimum. After inoculation, tubers were treated, then stored at 21 C for four days prior to disease evaluation. In Study 1, none of the formulations of P. fluorescens triculture significantly controlled leak compared to the inoculated control; however, the “fresh” triculture formulation significantly decreased leak incidence by 24% in Study 2. In all three studies the three-way fungicide mixture Azo+Flu+Dfz resulted in lower leak incidence (1, 7, and 24%, in studies 1, 2, and 3 respectively) compared with the inoculated control (12, 37, and 50% leak incidence, in studies 1, 2, and 3 respectively). Study 3 showed the formulations of P. fluorescens strains were not effective in leak control but when combined with Azo+Flu+Dfz leak incidence significantly decreased compared to the inoculated control. The lowest leak incidence was seen when the triculture component of the fungicide mix was rehydrated with Medium 1 in Study 3. The P. fluorescens triculture formulations mixed with Azo+Flu+Dfz and rehydrated with Medium 1 had slightly higher bacterial counts than the “fresh” triculture and the triculture dried on Kenite, potentially indicating a greater activity level afforded by cell revival in dilute culture medium. P. syringae, phosphorous acid and hydrogen peroxide-peroxyacetic acid were ineffective in controlling leak in all three studies. The efficacy of the Azo+Flu+Dfz mixture appears promising for the management of leak in storage. The efficacy of P. fluorescens tricultures used alone was inconsistent in limiting leak incidence (only significant in 1 of 3 studies), and additional research and development may be warranted to realize its potential as biological control agent.