Major fungal diseases
Brown rot, which is caused by species of the fungal genus Monilinia (M. fructicola, M. laxa, M. fructigena), is the most important post harvest disease of stone fruit. Depending on weather conditions and post harvest handling, there are other high-incidence post harvest diseases of stone fruits which cause molds and rots, whose pathogens are Botrytis cinerea, Geotrichum candidum, Rhizopus stolonifer, Mucor piriformis, Alternaria alternata and Penicillium expansum.
 1. Botrytis cinerea |  2. Geotrichum candidum |
 3. Rhizopus stolonifer |  4. Mucor piriformis |
 5. Alternaria alternata |  6. Penicillium expansum |
Effective postharvest decay control depends on an integrated approch based on proper pre-harvest and postharvest fungicide treatments, proper handling harvest practices, effective sanitation of fruit and facilities in the packinghouse, and proper storage and shipping conditions.
Researching for alternatives to fungicide
Postharvest fungicide treatments are usually fundamental to effectively control decay on stone fruits stored for long period before they are marketed. However, concerns about human health risks and environmental problems associated with fungicide residues have increased the need to find and develop alternatives to conventional fungicides. This need is stressed when fruits are exported to Countries where postharvest applications of conventional fungicides are prohibited.
During the last few years, post harvest research groups have evaluated aqueous solutions of many common food additives, GRAS (Generally Recognized As Safe) compounds, and low toxicity compounds as alternative treatments to control post harvest diseases of fruits and vegetables. The research results show that solution temperature, chemical concentration, and treatment duration for optimal decay control must be determined for each chemical and each host-pathogen system. Currently, very little research has been conducted to assess the antimicrobial activity of food additives and GRAS compounds against post harvest pathogens of stone fruit.The research of Palou et al.
Concerning this, Palou et al. (2009) have evaluated the effectiveness of a wide range of low toxicity compounds, mostly common food additives, for the control of the main post harvest pathogens of peach, nectarine, and plum. For the study, 5 cultivars of peach (Flavorcrest, O’Henry, Rich Lady, Ryan Sun, Last Chance), 2 cultivars of nectarine (Summer Fire, Spring Bright), and 2 cultivars of plum (Fortune, Royal Diamond) were used. The experiments were conducted over three consecutive seasons.
Flavorcrest peaches.
Stone fruits were stored at 1°C for 3 days before use. Fruits were surface disinfected, rinsed with fresh water, and left to air dry at room temperature before fungal inoculation. For the fungal inoculum, M. fructicola, B. cinerea, G. candidum, A. alternata, P. expansum, M. piriformis, and R. stolonifer were isolated. After fruit inoculation, fruits were stored at room temperature for 18 to 24 h before application of the antifungal treatment for spore germination to occur within the wound sites to simulate infections that occur during harvest.
For the study, 24 chemicals, mostly mineral salts and organic acid salts classified as food additives or as GRAS according to US and EU regulations, were tested.
All chemicals were purchased from Sigma-Aldrich Chemical Co. (St. Louis, MO). The research consisted of three phase. The first consisted of in vivo primary screenings of these chemicals, testing their effectiveness at 3 different concentrations against seven post harvest pathogens. A qualitative 4-point scale was established to assess the effectiveness of the treatment or their decay control ability in all tested fruit species:
- 0, no control;
- +, slight control (less than 50% reduction of disease incidence and/or severity compared to control fruits);
- ++ moderate control (more than 50% but less than 80%);
- +++ good control (more than 80% decay reduction).
Summer Fire nectarines.
After treatments, fruits were stored at 20±1°C and 90%±5% RU, after 3 and 7 days of incubations, disease incidence and severity and compound phytotoxicity were recorded. Based on the results of the previous experiments, the third research phase consisted in evaluating integrated treatments by combining antifungal food additives with synthetic fungicide, such as fludioxonil, to study the feasibility of new fungicidal mixture containing reduced doses of the conventional fungicide.
From in vivo primary screenings, 2-deoxy-D-glucose at 100 mM concentration, potassium carbonate at 250 mM concentration, sodium carbonate, sodium sorbate, potassium sorbate, and sodium benzoate at 200 mM concentration controlled postharvest diseases with an efficacy ranged from very good to moderate. Sodium and ammonium molybdates, lactic acid, and hydrogen peroxide had good decay control ability but they resulted unacceptable for their phytotoxic action on the skin of peach, nectarine, and plum.
However, the best compounds lacked effectiveness and persistence when tested against brown rot on fruits immersed for 60 seconds in acqueous solution at room temperature; potassium sorbate and sodium benzoate reduced brown rot incidence by less than 40%. Rinsing treated fruits with tap water reduced the efficacy of the compounds by up to 30%, while heating the solutions up to 55 or 60°C increased their efficacy. The efficacy of chemical compounds in aqueous solutions was less than treatments in aqueous solutions without additives. The authors have hypothesized that heated water alone is more effective than the low toxicity compounds to control post harvest decay. However, the application of heated solutions to stone fruits is still very limited due to the high risk of fruit injury.
The efficacy of tested chemical compounds differed significantly according to the pathogen, fruit species, cultivar, and external and physiological conditions of the stone fruit. Potassium sorbate and sodium benzoate resulted in the most effective compounds to control brown rot on nectarines ‘Spring Bright’ dipped for 60 seconds in aqueous solutions at room temperature, while none of the tested additives reduced this disease on ‘Flavorcrest’ peaches
All six tested compounds significantly reduced the incidence of sour rot on nectarines 'Spring Bright': sodium and potassium sorbate were the most effective chemical compounds in reducing disease incidence from about 85% on control fruits to 6 and 13%, respectively. The mixtures of fludioxonil with potassium sorbate and sodium benzoate were not effective to control brown rot, gray mold, and sour rot.
Results
The results of the research suggest that the potential use of low toxicity compounds to control the major postharvest diseases of stone fruits is currently limited. The authors inform that an additional work is in progress to characterize effective and safe postharvest hot water dips for California nectarines, peaches, and plums. The applications of heated aqueous solutions as postharvest treatment could be an useful alternative for handling organic fruits and for exporting California stone fruits productions to Countries where the use of postharvest conventional fungicides is prohibited.
Original study: Palou L., Smilanick J.L., Crisosto C.H., "Evaluation of food additives as alternative or complementary chemicals to conventional fungicides for the control of major postharvest diseases of stone fruit", 2009, Journal of Food Protection, Issue No. 72(5), pagg. 1037-1046. For more details: ucce.ucdavis.edu/files/datastore/234-1293.pdf