PLANT-GROWTH PROMOTING FUNGI FROM ZOYSIAGRASS RHIZOSPHERE AS POTENTIAL INDUCERS OF SYSTEMIC RESISTANCE IN CUCUMBERS

Sixteen fungal isolates from zoysiagrass rhizospheres that promoted th e growth of a variety of crop plants were tested for their ability to induce systemic resistance in cucumber plants against Colletotrichum o rbiculare, the anthracnose pathogen. Roots of cucumber plants were tre ated for different durations with these fungal isolates as barley grai n inoculum, mycelial inoculum, or culture filtrate. The induced plants were challenge inoculated with C. orbiculare at three different spore concentrations: 10(4), 10(5), or 10(6) spores per milliliter. Among t he 16 isolates, only 10 reduced the disease significantly and the prot ection obtained by these isolates varied among the methods of inducer application. However, isolates GU21-2, GU23-3, and GU24-3 produced the same effect when treated in all three forms and challenged with 10(4) spores per milliliter of the pathogen. Some isolates protected plants against high inoculum densities of the pathogen, while most of them w ere less effective under such pressure. The reduction in total lesion area and lesion number on leaves of plants induced by isolates GS6-1 a nd GU21-2 was similar to that induced by C. orbiculare. Among isolates that induced protection against anthracnose, isolates GS8-1, GS8-2, a nd GS8-3 colonized roots and significantly reduced the disease compare d with the protection caused by noninduced control. The other isolates that were unable to colonize roots also offered protection. The degre e of protection was highly dependent on the pathogen spore concentrati on and the inducer treatment period. A 24-h period was sufficient to i nitiate resistance; however, a 72-h duration proved to be more effecti ve. The induction of systemic resistance in cucumber plants against C. orbiculare might be due not only to root colonization, but also to tr iggering of the host defense mechanism(s) by certain factors produced by fungi and their metabolites. When provided as barley grain inocula, most of the isolates increased the plant height and biomass significa ntly. Treatment of cucumber seeds with the mycelial inocula and cultur e filtrates of certain isolates also caused promotion of height and bi omass of plants.