Abstract
Cucumber plants are often attacked by various pathogens, which can considerably decrease production and cause significant losses. One of the most prevalent fungal diseases is powdery mildew, caused by an obligate pathogen, Podosphaera xanthii. It is a serious disease that causes significant damage to the whole plant, i.e., leaves, fruits, and stems, under both greenhouse and field conditions. The main objective of this result is to assess the effectiveness of Bacillus spp. against cucumber powdery mildew under in vitro and in vivo conditions. Treatment with B. licheniformis and B. aerius culture filtrates reduced the conidial germination of the pathogen by 60 and 85%, respectively. Under greenhouse conditions, spraying cucumber plants with both microorganisms was effective at reducing powdery mildew disease severity. High reductions of disease severity were achieved by treatment of B. licheniformis as a cell suspension and B. aerius strain as culture filtrate, 45.3 and 77.3%, respectively, two days before inoculation. Additionally, treatment with these bacterial strains resulted in a significant increase in the fresh and dry weights of the cucumber plants. The highest increase of fresh and dry weight was found with B. licheniformis CS and B. aerius strain CF treatment at two days before or after infection. After treatment with the bioagents, the content of total phenols, polyphenol oxidase, and peroxidase was enhanced in treatment plants. The use of B. licheniformis and B. aerius as foliar sprays significantly induced resistance to P. xanthii in cucumber plants and stimulated many biochemical functions. Therefore, we propose B. licheniformis and B. aerius as an effective alternative to harmful chemicals.