natural-products-in-plant-pest-management

246 M. Santos et al.predominate in grape pomace (high sugar, low cellulose) in suppressingS. rolfsii. In this medium, Trichoderma spp. were absent and would have beenineffective even if applied. Rhizoctonia (a common nursery pathogen) is alsoa very active saprophyte in fresh wastes but not in low-cellulose mature composts.Trichoderma, by contrast, colonizes both. However, the hyperparasiticactivity and antibiotic production by Trichoderma is repressed in fresh compostbecause of the high glucose level. Saline composts (for instance overfertilizedmedia) enhance Pythium and Phytophthora, whereas composts withlow C/N ratios that release nitrogen, especially as ammonia, promotes thegrowth of Fusarium on host species. The appropriate control of the pottingmedium with a pH greater than 5.0 and moisture content (oven dried)maintained between 40 and 50% is also important (Hoitink et al., 1997).12.2 Competition Among Microbial PopulationsMicrobiostasisThere is a competition for nutrients in exudates such as sugars and aminoacids which leak out of seeds during germination or out of root tips as plantsgrow through the soil. Pathogens that grow or swim to these sources of nutrientsmust compete with the beneficial microflora at the infection site on thesurface of the seed or root (Hoitink and Changa, 2004). This type of competitionplays a major role in general suppression and with ‘nutrient-dependent’pathogens such as Pythium and Phytophthora species. The competition includesmicrobial competition for nutrients and competition for infection sites androot colonization. Limited nutrient availability to the germinating spores orinvading hyphae is the most common explanation given for the microbialcause of fungistasis. On the other hand, fungistasis has been attributed to thepresence of antifungal compounds of microbiological origin. The productionof antifungal compounds is correlated with available carbon, which is notsurprising, as microbial production of antibiotics can be induced by interspecificcompetition for substrates (Ellis et al., 2000; Slattery et al., 2001). The sensitivityof plant pathogenic fungi to fungistasis is thought to protect themfrom germinating and initiating growth under unfavourable conditions (e.g.,the absence of a host plant) (Garrett, 1970; Lockwood, 1977). However, thisprotection comes at a cost, since the viability of resting structures decreasesduring prolonged incubation in soils (Lockwood, 1986). The negative effectsof fungistasis on the inoculum density of plant pathogenic fungi has beensuggested as a mechanism to explain the commonly found correlationbetween fungistasis and disease suppressiveness (Lockwood, 1977; Hornby,1983; Larkin et al., 1996; Knudsen et al., 1999).Iron competition also plays a major role in nutrient competition amongpathogens and beneficial microorganisms in infection sites (Hoitink andChanga, 2004). Ionic forms of iron, especially iron (III), its most common state,are very insoluble, especially when plants are grown on calcareous soils orsoils with high pH. Iron is only available to organisms at concentrations at or