Plant Growth Promoting Fungi from Zoysiagrass Rhizosphere as Potential Inducers of Systemic Resistance in Cucumbers

Abstract

Sixteen fungal isolates from zoysiagrass rhizospheres that promoted the growth of a variety of crop plants were tested for their ability to induce systemic resistance in cucumber plants against Colletotrichum orbiculare, the anthracnose pathogen. Roots of cucumber plants were treated for different durations with these fungal isolates as barley grain 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 the 16 isolates, only 10 reduced the disease significantly and the protection 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 were less effective under such pressure. The reduction in total lesion area and lesion number on leaves of plants induced by isolates GS6-1 and GU21-2 was similar to that induced by C. orbiculare. Among isolates that induced protection against anthracnose, isolates GS8-1, GS8-2, and GS8-3 colonized roots and significantly reduced the disease compared with the protection caused by noninduced control. The other isolates that were unable to colonize roots also offered protection. The degree of protection was highly dependent on the pathogen spore concentration and the inducer treatment period. A 24-h period was sufficient to initiate resistance; however, a 72-h duration proved to be more effective. The induction of systemic resistance in cucumber plants against C. orbiculare might be due not only to root colonization, but also to triggering 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 significantly. Treatment of cucumber seeds with the mycelial inocula and culture filtrates of certain isolates also caused promotion of height and biomass of plants