BIOCONTROL POTENTIAL OF Trichoderma harzianum Rifai, (1969) AND Beauveria bassiana (Bals.-Criv.) Vuill. (1912) AGAINST Phytopthora infestans AND Alternaria solani CAUSING BLIGHT IN TOMATO

Abstract

The most devastating fungal pathogens in tomato (Solanum lycopersicum L.) production are Alternaria solani and Phytophthora infestans which cause early blight and late blight diseases respectively. The pathogens thrive well in humid and wet conditions that make it difficult for fungicide application. Tomato is a popular vegetable that is grown mainly in small holder farms for home consumption and local market with many benefits ranging from nutrition to source of income. It is grown in 95% open fields and 5% greenhouse. This study sought to find alternative control to these pathogens that employs the use of Trichoderma harzianum and Beauveria bassiana. T. harzianum has been widely used against plant pathogens whereas B. bassiana has been rarely used. T. harzianum was isolated selectively from soil using Potato Dextrose Agar whilst B. bassiana was isolated from a dead beetle by use of corn meal agar. Both pathogens were isolated from diseased tomato plant tissues showing the symptoms of early and late blight. Both the potential biocontrol agents and the pathogens were identified using cultural, morphological and microscopic characteristics. In vitro tests of antagonism by dual culture technique, volatiles and non-volatiles production of each biocontrol agent (BCA) were performed. In vivo tests were performed to compare the efficacy of the two BCAs. Daily growth recording of the pathogen colonies under different inhibitory treatments were taken for seven days. Dual culture of T. harzianum and A. solani resulted in a significant reduction in the colony growth of the pathogens. Subjecting the seventh day recordings to t-test resulted in a statistically different result at P < 0.001. There was a growth reduction of about 65% in the case of P. infestans. Treatment with volatile metabolites produced by T. harzianum also resulted in inhibition of A. solani colonies by 48.25% (P =0.00034), While it also significantly slowed P. infestans colonies growth by 38.7%. Trichoderma harzianum culture filtrate (non-volatile metabolites) however was not effective against A. solani colonies. The colonies' diameter did not significantly differ from that of the controls (p > 0.05). As for P. infestans however, the highest concentration of the culture filtrate (15% v/v), showed stronger inhibition albeit weak (control; 70.33mm, 5%; 70mm, 10%; 69.66mm and 15%; 62mm). Beauveria bassiana on the other hand was not effective in dual culture against A. solani but there was a very clear zone of inhibition against P. infestans. Its volatiles reduced the growth of A. solani by 22.5% while that of P. infestans was reduced by 45.9%. The culture filtrate proved effective against A. solani (10%v/v; 33%, 15%v/v; 38.88%) while for P. infestans, none of the concentrations effectively inhibited the colonies. Greenhouse experiments utilized spore suspensions of 3×108 conidia ml-1. It was revealed that B. bassiana was more effective in the reduction of disease severity. T. harzianum reduced disease severity by 25% and 28.75% against A. solani and P. infestans respectively. Beauveria bassiana reduced the disease score against A. solani by 50% while against P. infestans by 46.25%. The biocontrol agents used in the current study were therefore effective against the test pathogens. More experiments ought to be carried out to establish the extent of potency of B. bassiana against phytopathogens.