IMPROVEMENT OF GROWTH AND BIOCONTROL ABILITY OF EPICOCCUM NIGRUM UNDER WATER STRESS CONDITIONS
IMPROVEMENT OF GROWTH AND BIOCONTROL ABILITY OF EPICOCCUM NIGRUM UNDER WATER STRESS CONDITIONS S PASCUAL 1, 2, N MAGAN 2 and P MELGAREJOl 1 Plant Protection Department, INIA, Crta. Coruha Km. 7, 28040 Madrid, Spain; 2 Applied Mycology Group. Biotechnology Centre. Cranfield University. Bedford MK43 OAL, UK Background and objectives Epicoccum nigrum, a component of the resident mycoflora of peach twigs and flowers, reduces twig blight caused by Monilinia laxa. However, biocontrol obtained is variable, depending on disease severity and humidity conditions . Water availability is one of the most important limiting environmental factors for microbial growth in the phyllosphere. An improvement in the ability of microbial antagonists to grow under conditions of water stress could enhance their ecological competence, leading probably to improved biocontrol. The present investigation attempted: a) to improve the ability of E.nigrum to grow and colonise dry environments, by physiological manipulation, and b) to improve biocontrol of the disease by the modified inoculum of E.nigrum. Materials and methods Germination and germ tube growth of E.nigrum under freely available and water stress conditions: Spores were produced at high (0.996) and reduced water activity (aw) (0.98) on PDA plates. Spores were plated on water agar or water agar at 0.935 aw. Plates were incubated at 25oC and germination and germ tube extension were determined at different sampling times during 23 h. Growth rates: Growth rates of E.nigrum spores produced at 0.996 and 0.98 aw were determined at 10, 17 and 25oC. Culture media used were: PDA, 1110 strength PDA and a minimal salts medium (MM). Medium aw was modified to 0.996 (unmodified), 0.99, 0.984 and 0.95 aw. Disease control: Suspensions of conidia+mycelium of E.nigrum produced at 0.996 and 0.98 a, were sprayed onto peach twigs one day before artificial inoculation with M.laxa. Applications were repeated four times. Disease was assessed by an index rating the appearance of the twig. Results and conclusions Germination and germ tube growth of E.nigrum under freely available and water stress conditions: Spores produced at 0.996 aw germinated faster than those produced at 0.98 aw in media with freely available water (0.996 aw) and under water stress conditions (0.935 aw). However, growth of germ tubes showed a different pattern: at 0.935 aw germ tube extension from spores produced at 0.98 aw was significantly longer than those from spores produced at 0.996 aw, after both 12 and 23 h incubation. Growth rates: Maximum growth rates were obtained at 170C and 0.996 aw for both types of inoculum. Spores produced at 0.98 aw has higher growth rates than those produced at 0.996 aw in some of the conditions tested. This occurred under water stress conditions: for example on PDA at 0.99 aw and 10 or 250C, or on 1/10 PDA or MM at 0.984 and different temperatures. Spores produced at 0.98 aw also had faster growth under conditions of freely available water in some cases (for example on PDA or 1/10 PDA at 10oC). However, spores produced at 0.996 had generally higher growth rates than those produced at 0.98 when water was freely available (0.996 aw). It was notable that colonies from spores produced at 0.98 aw caused a more intense pigmentation of the culture medium in some of the conditions tested. In other cases sporulation was heavier. These results indicate a higher tolerance of the inoculum produced at reduced aw to colonise dry environments. Disease control: Disease control was higher when the fungus was produced at reduced aw (0.98) compared to high aw (0.996), although neither differed from captan. Improved ecological competence and biocontrol ability are probably related to an enhance accumulation of compatible solutes , which makes the fungus less dependent on high atmospheric RH to become established in the phyllosphere. References 1. Madrigal C, Pascual S, Melgarejo P, 1994. Plant Pathology 43, 554-561. 2. Pascual S, Magan N, Melgarejo P, 1996. Proceedings BCPC: Pests and Diseases, pp.410-411.