Control of Postharvest Blue Mold of Apple

Postharvest decay in apple is caused by two diseases, blue mold (causal agent, Penicillium expansum) and gray mold (causal agent, Botrytis cinerea). Blue mold is more damaging than the gray mold. To determine the cause of this increase in post harvest decay a study has been initiated to identify if the P. expansum has developed resistance to MERTECT (thiabendazole; TBZ) and diphenylamine (DPA). (Figure 1)

Figure 1. Untreated control (left) versus apple treated with Scholar fungicide (right). Treatment applied within one day after wounding.

It was observed that some of the isolates collected from the apple storages were TBZ-resistant. To overcome the fungicide resistance problems, during 2001- 2002, a low risk phenylpyrrole fungicide, SCHOLAR (active ingredient, fludioxonil), was tested for efficacy against blue mold on "Empire" and "Red Delicious" apples. Surface disinfected apple cvs. Empire and Red Delicious, were wounded and co-treated with recommended concentrations of TBZ, and four to six different concentrations of SCHOLAR fungicide and with 1 x 10 4 conidia/ml of P. expansum inoculum by dip or drench methods. Treated apples were incubated for 3 months under controlled atmosphere storage and also 4 °C for 2 months. Results have shown that SCHOLAR reduced blue mold by 100% at a concentration of 200, 300 and 600 ppm ai of fludioxonil/ml as compared to the untreated controls. The treatments were effective if the fungicide was applied within a day of wounding. TBZ which gave excellent control (100%) on blue mold caused by TBZ-sensitive Penicillium, but it reduced blue mold caused by TBZ-resistant Penicillium by only 35% and 4% at 4°C and 20°C, respectively (Figure 2). The treatments were also tested with DPA and all the concentrations gave 100% of control indicating that there was no negative reaction between SCHOLAR and DPA. In summary the SCHOLAR gave good control of blue mold of apple during storage under CA and also cold storage conditions. In these experiments, the fungicide, SCHOLAR was used experimentally, and is not registered for post harvest treatment on apples in Canada and cannot be used commercially.

Figure 2. Control of TBZ sensitive & TBZ resistant Penicillium by SCHOLAR & MERTECT.