Postharvest control of Monilinia laxa and Rhizopus stolonifer in stone fruit by peracetic acid

Abstract

Peracetic acid (PAA) treatment of stone fruit (sweet cherry, apricot, peach and nectarine) reduced the incidence of brown rot caused by Monilinia laxa and soft rot caused by Rhizopus stolonifer. The efficacy of the treatment depended on the length of time. Fruit, neither wounded nor inoculated and dipped for 1 min in a 125 mg L⁻¹ PAA solution, showed a significant reduction of Monilinia rots with respect to control. Significant inhibition was also observed on fruit wounded and inoculated with R. stolonifer and treated for 1 min with 250 mg L⁻¹ PAA solution. Sodium bicarbonate (SBC), sodium propionate (Na-Pro) and potassium sorbate (K-Sorb), substances generally regarded as safe (GRAS), were also evaluated. Phytotoxic effects appeared on fruit treated with SBC at the minimum effective concentration (3%). Any rot reduction was observed in Na-Pro fruit treated, only K-Sorb at 1.5% was able to significantly reduce Monilinia infections in sweet cherries (61.6%), apricots (78%) and nectarines (31.8%) with respect to the controls, without any visible damage on the skin. Similar results were obtained on apricot wounded and inoculated with R. stolonifer. Fruit hydro-refrigeration significantly reduced the incidence of brown rot in Nero I and Van sweet cherries; disease control was improved by addition of PAA (125 mg L⁻¹) in cold water. PAA efficacy on pre-existing infections can be very useful to control stone fruit diseases that can spread during shipping and marketing.

Introduction

Fruit injured during harvesting or handling may come into contact with pathogens when it is packed, stored or shipped. The pathogens are wound parasites and in most cases require a wound in the skin or stem to enter into contact with susceptible tissue and initiate infection (Spotts et al., 1998). Infection occurs during harvest or when fruit moves through water in the packing-line dump tank, to be washed, cleaned and rapidly refrigerated. Recirculated-water quickly becomes heavily contaminated by fungal spores and may infect dipped fruit.

Brown rot caused by Monilinia laxa (Aderh. and Ruhl.) is the most important stone fruit decay in Europe; the pathogen can only be controlled by fungicide spray programs in the fields, since, in Italy and Spain, postharvest treatments are not allowed. For this reason, if weather conditions are favourable to brown rot during the postharvest phase, important economic losses in sweet cherries, apricots, peaches and nectarines can occur.

Soft rot caused by Rhizopus stolonifer (Ehrenb.: Fr.) Vuill appears after storage, particularly in the market or in the consumer’s home when temperatures are higher than 5 °C. The pathogen is not efficiently controlled by registered fungicides and when fruit are mature or processed at room temperature, soft rot spreads quickly from infected to healthy fruit (Ogawa et al., 1995).

In the attempt to reduce the incidence of brown and soft rot by alternative methods to fungicides, interest in safe, effective and economical substances like food additives or sanitizing products has greatly increased (Prusky et al., 2001, Palou et al., 2002a, Palou et al., 2002b). Among these, chlorine, peracetic acid (PAA) or ozone are considered quite promising. When washing water is exposed to sanitizing products, spores suspended in water or present on the fruit surface are killed, with a noticeable reduction in inoculum level and a consequent decrease in disease incidence (Barkai-Golan, 2001). The effectiveness of chlorine in decay prevention has been known for some time (Baker and Heald, 1932), although with several limitations, such as the rapid drop of fungistatic activity in the presence of organic substances. On the other hand, limited data are available on the control of postharvest fruit decay by PAA. It has been tested on degreened orange (Brown, 1987) and on stone fruit artificially infected by M. laxa. In a previous paper, under semi-commercial conditions brown rot was totally controlled when conidia remained in contact with PAA (250 mg L⁻¹) for 5 min (Mari et al., 1999).

The potential of bicarbonate salts in controlling postharvest pathogens has been demonstrated on a wide range of species: sweet cherries (Karabulut et al., 2001), apples and peaches (Droby et al., 2003), and melons (Aharoni et al., 1997), alone or in combination with biocontrol agents.

The present study was conducted to evaluate: (1) the effects of PAA on stone fruit naturally infected by M. laxa; (2) the efficacy of PAA in the control of brown rot on hydro-refrigerated sweet cherries; (3) the effects of PAA on stone fruit artificially infected by R. stolonifer; and (4) to compare the efficacy of PAA to salts, widely used as preservative materials in the food industry, in the control of brown and soft rot.