عنوان مقاله [English]
Lettuce (Lactuca sativa L.) from the Asteraceae family is one of the most important vegetables due to its rapid growth and commercial value. Currently, the market share of organic vegetables is constantly increasing due to customer demand for safer and healthier food. Excessive use of pesticides and chemical fertilizers threatens the environment and leads to the production of unsafe food products. Therefore, it is important to find alternatives instead of using pesticides chemical methods to manage powdery mildew. Generally, biotic and abiotic stresses are among the factors that have a destructive effect on growth and development, performance, and production of plant biomass. Fungicides can be the most effective method of controlling the powdery mildew disease, but this pathogen can develop resistance to fungicides. Rhizosphere bacteria are among the living agents that, by producing some allelochemicals, cause the dissolution of soil nutrients, increase the availability of nutrients, and induce plant resistance to biotic and abiotic stresses. In addition, they enhance host plant growth through an indirect mechanism, including the inhibition of disease-causing pathogens by releasing some allelochemical substances. The biological control of powdery mildew disease with the use of rhizospheric bacteria in lettuce and zucchini has been reported.
Material and Methods:
To evaluate the biological control of powdery mildew fungus with plant growth promoting rhizobacteria (PGPR) and effects on yield and quality of New Red Fire greenhouse lettuce, the experiment was carried out in a completely randomized design with three replications in the Research greenhouse of University of Zanjan during 2020. Experiment treatments consisted of five levels of PGPR (Pseudomonas vancouverensis- VPM, Pseudomonas Koreensis- KPM, Pantoea agglomerans- PAPM, Pseudomonas putida- PPM, and one level of combined bacteria (Pantoea agglomerans+ Pseudomonas Koreensis+ Pseudomonas putida+ Pseudomonas vancouverensis- MBPM, one level of chemical fertilizer 100% N, P and, K according to soil test results- NPK, and two control treatment without powdery mildew condition (C) and under powdery mildew conditions (CPM).
The “New Red Fire” lettuce seeds were surface sterilized with 0.5% (v/v) sodium hypochlorite for 10 min and germinated at 20ºC. After germination, seedlings with similar size were transplanted into pots. Plants were grown under greenhouse condition with 60/70 % (day/night) relative humidity, 15/18 °C (day/night) temperature. Inoculation of pathogenic fungi was done 40 days after seed germination. Plants were harvested after 75 days. The chlorosis and necrosis spots number on each plant, plant fresh weight, plant dry weight, leaf number, total chlorophyll, total phenol and flavonoids contents, antioxidant activity, anthocyanin content, and catalase and peroxidase enzyme activity were measured.
The results showed that the application of potassium and phosphorus solubilizing bacteria and NPK fertilizer significantly increased plant growth compared to control plants under the stress of powdery mildew fungus. The highest plant fresh weight, percentage of plant dry weight, and leaf number were obtained with the application of combined potassium and phosphorus solubilizing bacteria treatment and 100% N fertilizer under the biostress. The maximum total chlorophyll was obtained with the application of Pseudomonas koreensis and Pantoea agglomerans bacteria. 100% reduction of necrosis spots was obtained by using the Pantoea agglomerans bacteria. The maximum of chlorosis spots (increase of 55.8%) and necrosis spots (an increase of 88.8%), total phenol (an increase of 52%), total flavonoids (an increase of 39.3%), catalase (an increase of 28.4%) and peroxidase enzymes activity (49.1%) were obtained with application of NPK fertilizer. No significant effect on antioxidant activity was observed with the application of chemical fertilizer and rhizosphere bacteria under the Biostress. The maximum anthocyanin contents were obtained with the application of Pseudomonas koreensis.
According to the results, the application of NPK chemical fertilizer and seed pretreatment of lettuce with PGPR increased the value of antioxidant compounds including total phenol, flavonoid, and anthocyanin contents and catalase and peroxidase enzymes activity under powdery mildew conditions. Inoculation of lettuce seeds with PGPR, in addition to improve plant growth under biological stress conditions, increased anthocyanin contents and induced the resistance of lettuce plants to powdery mildew. Seed pretreatment with PGPR reduced chlorosis and necrosis spots in leaves. Therefore, pretreatment of lettuce seeds with PGPR instead of chemical compounds (fertilizers, pesticides and plant growth regulators) is recommended to improve the yield and quality of lettuce under powdery mildew conditions.
Key Words: Anthocyanin, Biological control, Biostress, Necrotic spots, Powdery mildew.