Evaluating the Ability of some Medicinal Plants for Controlling Rhizopus (Rhizopu snigricans) and Black Spot Rot (Alternaria alternate) as Postharvest Diseases in Tomato Produced under Conventional and Organic Cropping Systems

After crops harvesting, conditions and durations of storage are considered as the most crucial factors formaintaining the nutritional value and quality of agro-horticultural products such as tomato (Lycopersicom esculentum Mill.) and its waste reduction. However, the rhizopus rot (Rhizopus stolonifer) and black spot rot (Alternaria alternate) are the most important postharvest diseases in tomato during storage. In other word, among the factors reducing quality of the postharvest tomato, Rhizopus nigricans Ehrenb. (Rhizopus stolonifer) and Alternaria alternate (Fr.:Fr.) Keissl. f. sp. lycopersici paly a special role in the contaminated tomato fruits that can affect its taste, firmness and stiffness.
In recent years, due to the problems and threats arising from the use of chemical fungicides in agricultural systems, principled management of alternative biological approaches for reducing the postharvest contamination in tomato, especially during storage, is emphasized more than ever.
Considering these conditions, the current study was aimed to investigate the effects of some medicinal plants including thyme (Thymus vulgaris L.), pennyroyal (Mentha pulegium L.), peppermint (Mentha piperita L.), eucalyptus (Eucalyptus globules L.), caster bean (Ricinus communis L.) and tomato in their ability to control the rhizopus (Rhizopus nigricans) and black spot rot (Alternaria alternate) in tomato production under conventional and organic cropping systems.
The experiment was conducted at Faculty of Agriculture, Ferdowsi University of Mashhad, Iran, during theyear of 2010. A completely randomized design was used based on factorial arrangement with three replications and 14 treatments. Two cropping production systems (conventional and organic) and seven medicinal plants (thyme, pennyroyal, peppermint, eucalyptus, caster bean, tomato and control) were the first and the second experimental factors, respectively.
After collecting plant samples from the research farm of Ferdowsi University of Mashhad, each sample was separately placed in cardboard package and then was transferred to governesses. By observing the disease sign during storage, the tomato fruits were graded based on the severity of postharvest diseases (the rhizopus and black spot rot) on tomato as follow: First- grade (no disease), second- grade (observation of very low disease sign in fruits), third- grade (observation of low disease sign in fruits, low watery fruits), fourth- grade (observation of disease sign in one third of surface fruits, moderate watery fruits), and fifth- grade (observation of disease sign more than one third of surface fruits, high watery fruits).
For statistical analysis, non-parametric Kruskal-Wallis test were performed using MINITAB software.
Based on Kruskal-Wallis nonparametric test, organic and conventional cropping systems had different effects on storage- rot rate of tomato. In organic cropping systems, storage- rot rate (1.48 fruit per week) significantly decreased by 20%, compared to conventional cropping systems. In this regard, it has been reported that the soil fertility and applying organic fertilizer based on implementing organic farming systems has an important role in increasing the durability, nutritional value and quality of postharvest tomatoes.
According to the results, medicinal plants had significant effects on storage- rot rate in tomato. Results indicated that the medicinal plants significantly reduced storage rhizopus and black spot rot in tomato, except caster bean and tomato. Among the treatments, peppermint and pennyroyal considered as the most effective plants in reducing the storage- rot rate and increasing the rigid and healthy fruits; so that the use of these plants in a tomato packing significantly decreased the storage- rot rate by 21%, compared to control treatment.
As it can be seen from the results, a significant decrease in symptoms of fungal diseases and an increase in the number of healthy postharvest fruits due to application of mentioned plants can demonstrate the ability of these plants in reducing the activity of fungal pathogens during tomato storage. In line with the results, Feng & Zheng (2007) reported that the application of essential oil of thyme can play an effective role in controlling growth of fungal pathogens in tomato through reducing sporesgermination ability.
In conclusion, the results underlined the role of organic systems in improving the quality of postharvest tomatoes, compared with conventional systems. Moreover, for controlling the fungal pathogens, bio-products derived from medicinal plants can be especially considered in line with processing the postharvest tomatoes.