Production, survival, and evaluation of solid-substrate inocula of Penicillium oxalicum, a biocontrol agent against Fusarium wilt of tomato

Abstract

Production of conidia of Penicillium oxalicum (ATCC number pending), a biocontrol agent of Fusarium oxysporum f. sp. lycopersici, was tested in liquid and solid fermentation. P. oxalicum produced 250-fold more conidia in solid than in liquid fermentation at 30 days after inoculation of substrate. Solid fermentation was carried out in plastic bags (600 cm3) especially designed for solid fermentation (VALMIC) containing 50 g of peat/vermiculite (PV) (11, wt/wt) with 40% moisture, sealed, sterilized, and then inoculated with 1 ml of a conidial suspension of P. oxalicum (105 conidia g-1 dry substrate), sealed again, and incubated in darkness at 20 to 25°C for 30 days. Addition of amendments to PV in a proportion of 0.5 (wt/wt) significantly increased conidial production of P. oxalicum. The best production was obtained on PV plus meal of cereal grains (barley) or leguminous seeds (lentil) (100-fold higher). Conidial production obtained after 5 days of inoculation was similar to that obtained at 30 days. However, viability of conidia produced in PV plus lentil meal was 35% higher than that of conidia produced in PV plus barley meal. Changes in proportions (110.5, wt/wt/wt; 111, wt/wt/wt; 10.50,5, wt/wt/wt; 1:1:0.5, vol/vol/vol) of components of the substrate (peat/vermiculite/lentil meal) did not enhance production or viability of conidia. Optimal initial moisture in the substrate was 30 to 40%. At lower moistures, significant reductions of production of conidia were observed, particularly at 10%. There was a general decline in the number of conidia in bags with time of storage at -80, -20, 4, and 25°C, or at room temperature (range from 30 to 15°C), with the highest decline occurring from 60 to 180 days. Conidial viability also was reduced with time, except for conidia stored at -20°C. Fresh conidia produced in solid fermentation system or those conidia stored at -20°C for 180 days reduced Fusarium wilt of tomato by 49 and 61%, respectively.