Development of an integrated management approach to controlling bacterial speck of tomato.
Ncwane, Nonduduzo Charity.
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Bacterial speck of tomato caused by Pseudomonas syringae pv. tomato (Pst) is an economically important bacterial diseases in many tomato growing regions worldwide. The development of bacterial speck epidemics is favoured by cool temperatures, high humidity and prolonged leaf wetness. As a result of infection, dark-brown to black coloured lesions surrounded by halos that eventually lead to premature defoliation are observed. Yield reduction results from the reduced photosynthetic capacity of infected leaves, resulting in flower abortion. Infected tomato fruit become unattractive and unsuitable for sale on the fresh market or for processing. In this study 250 bacterial and 100 yeast isolates were obtained from diseased and healthy tomato leaf samples. These were screened in vitro for activity against Pst. Thirty bacterial and 20 yeast isolates demonstrated significant inhibition of Pst. During the secondary in vitro screening, 10 bacterial and 7 yeast isolates successfully inhibited the growth of Pst on tryptone soy agar (TSA) plates and were selected for further studies under greenhouse conditions. Bacterial Isolates LN17, LN24 and LN10 showed clear zones of inhibition against Pst ranging from 26–29 mm in diameter. During in vitro screening, seven yeast isolates were selected, based on their ability to reduce the development of bacterial speck lesions on tomato leaves over a period of 7 d using a detach-leaf technique. Yeast Isolates IB7, Y54 and Y21 moderately suppressed bacterial speck lesions and were rated Class 2 on a five class rating system. Isolate Y25 was the best isolate and was rated Class 1. Scanning electron microscopy revealed that yeast cells colonised the leaf surface. Ten bacterial and seven yeast isolates selected from in vitro screening were further screened under in vivo under greenhouse conditions for their ability to control bacterial speck of tomato. Two of the bacterial isolates were identified as Bacillus cereus; one as B. thuringiensis; 5 as Bacillus spp.; and 2 as unidentified Bacillus spp. Two of the yeast isolates were identified as Rhodotorula glutinis; two as Rhodotorula mucilaginosa; and the remaining three isolates as Cryptococcus magnus, C. diffluens and Rhodosporidium babjevae, respectively. Bacterial isolate Bacillus thuringiensis LN1 and Bacillus sp. LN10 reduced AUDPC units by 25 and 52% and by 51 and 48% in Experiments 1 and 2, respectively compared to the pathogen inoculated control. Yeast isolates Rhodotorula glutinis Y25 and Bacillus cereus Y14 caused reductions in AUDPC units by 95 and 86% and 42% and 58% in Experiments 1 and 2, respectively, compared to the pathogen inoculated control. Based on the results from the greenhouse studies, two isolates (one bacterial and one yeast) were selected for further studies under nursery conditions. These two isolates were Bacillus sp. Isolate LN10 and R. glutinis Isolate Y25. Reduced concentrations of a plant activator, acibenzolar-S-methyl (ASM) and a plant sanitiser didecyl-dimethyl-ammonium chloride (DDAC), were evaluated for their effect on bacterial speck under greenhouse conditions in an effort to use them together with the two selected biological control agents as an integrated strategy to manage bacterial speck. Treatment with 25% of the recommended concentration of acibenzolar-S-methyl caused significant disease suppression (81.2% control) in both Experiments 1 and 2. Treatment with 25% of the recommended concentration of didecyl-dimethyl-ammonium chloride reduced disease severity by 6.3% and 9.3% in Experiments 1 and 2. The best control strategies were selected for integrated disease management studies under greenhouse and nursery conditions. Two biological control agents, Bacillus sp. LN10 and R. glutinis Y25, the plant sanitizer DDAC (at 25% of recommended strength) and the plant defence inducer ASM (at 25% of recommended strength) were used. It was found that any combination with 25% ASM caused significant disease reduction. 25 % ASM + Bacillus sp. LN10 + R. glutinis Y25 had a synergistic effect and gave disease reduction of 99.1% and 92.62% under greenhouse and nursery conditions, respectively, and was more effective than the copper bactericide control. All combinations with 25% DDAC provided no significant disease control. The combination treatment of 25% DDAC + Bacillus sp. LN10 + R. glutinis Y25 was ineffective under greenhouse and nursery conditions. The combination treatment of the two biological control Bacillus sp. LN10 + R. glutinis Y25 was ineffective under greenhouse and nursery conditions.