Powder Formulations of Pseudomonas fluorescens to Control Pigeonpea Wilt P. Vidhyasekaran, K. Sethuraman, K. Rajappan, and K. Vasumathi Department of Plant Pathology, Tamil Nadu Agricultural University, Coimbatore 641 003, India Received May 21, 1996; accepted January 24, 1997 Pseudomonas fluorescens strains which effectively inhibited mycelial growth of Fusarium udum, the pi- geonpea (Cajanus cajan) pathogen, were isolated from the rhizoplane of different crops. Various powder for- mulations of two efficient P. fluorescens strains were developed. All freshly prepared powder formulations were effective in controlling the disease, but their efficacies varied depending upon the length of storage. Talc formulations were effective even after 6 months of storage, while peat formulations were effective up to 60 days of storage. The shelf life of vermiculite, lignite, and kaolinite formulations was short. Unformulated bacterial suspensions could not be stored even for 10 days, at which time their efficacy was completely lost. The bacterial strains survived in pigeonpea rhizo- sphere throughout the crop-growth period. The talc- based powder formulations effectively controlled pi- geonpea wilt and increased yield in two field trials. Development of powder formulations of P. fluorescens will aid large-scale application of biological control in farmers’ fields. r 1997 Academic Press KEY WORDS: Pseudomonas fluorescens; Fusarium udum; pigeonpea wilt; powder formulation; shelf life; biocontrol. INTRODUCTION Pigeonpea (Cajanus cajan (L.) Millspaugh) is one of the major grain legume crops grown in Southeast Asia, Africa, and the Americas (Nene et al., 1990). A wilt disease caused by Fusarium udum Butler is the most important disease of the crop, causing yield loss of about U.S. $36 million in India and U.S. $5 million in Africa per annum (Kannaiyan et al., 1984). The fungus is soil-borne (Reddy et al., 1990). Chemical control is impractical and high-yielding, disease-resistant crop varieties are lacking (Reddy et al., 1990). Fusarium wilt diseases have been reported to be controlled by certain strains of fluorescent pseudomonads (Gamliel and Katan, 1993; Lemanceau et al., 1993; Sneh et al., 1984; Tari and Anderson, 1988; Weller and Cook, 1986). However, in all these studies the bacteria have been used as cell suspensions to control the disease. This method of application is impractical for large-scale field use due to difficulty in handling, transport, and stor- age. In the present studies we isolated several strains of Pseudomonas fluorescens Migula from the rhizo- sphere of different crops, screened for their efficacy in inhibiting the growth of F. udum in vitro, developed powder formulations of the effective strains, and demon- strated their efficacy in controlling pigeonpea wilt under greenhouse and field conditions. The practical utility of these formulations for biological control of the disease is described in this report. MATERIALS AND METHODS Isolation of Pathogen and Fluorescent Pseudomonads An isolate of Fusarium udum was obtained from in- fected pigeonpea stems and roots using potato dextrose agar (PDA) medium. The pathogenicity of the fungus was tested by multiplying the fungus in sand–maize medium (Riker and Riker, 1936). Four pigeonpea seeds (cv Co5) were sown in each pot (300 mm in diameter) containing sterilized soil incorporated with the fungal culture at a ratio of 1:19 (sand–maize inoculum:soil). The wilt incidence was assessed and reisolation of the pathogen from the wilted plants was done using PDA. Rhizoplane-colonizing fluorescent pseudomonads were isolated from fresh roots of pigeonpea, chickpea, and rice plants from several geographic areas and soil types. After vigorous shaking of excised roots to remove all but tightly adhering soil, root segments (1 g) were shaken in 100 ml of sterile distilled water for 15 min and serial dilutions were made. Fluorescent pseudo- monads were isolated using King’s medium B (KMB; King et al., 1954) and fluorescent colonies were de- tected by viewing under UV light. Efficacy of Fluorescent Pseudomonads Efficacy of the fluorescent pseudomonad isolates was tested by streaking the bacteria at one side of the petri BIOLOGICAL CONTROL 8, 166–171 (1997) ARTICLE NO. BC970511 166 1049-9644/97 $25.00 Copyright r 1997 by Academic Press All rights of reproduction in any form reserved.