Lack of induced systemic resistance in peanut to late leaf spot disease byplant growth-promoting rhizobacteria and chemical elicitors

A disease assay was optimized for late leaf spot disease of peanut using Ce rcosporidium personarum in the greenhouse, and this assay was used in attem pts to elicit induced systemic resistance using strains of plant growth-pro moting rhizobacteria (PGPR) and chemical elicitors. Nineteen strains of spo re-forming bacilli PGPR, including strains of Paenibacillus macerans, Brevi bacillus brevis, Bacillus laterosporus, B. subtilis, B. pumilus, B. amyloli quefaciens, B. sphaericus, B. cereus, and B. pasteurii, which previously el icited systemic disease control activity on other crops, were evaluated in greenhouse assays. Seven PGPR strains elicited significant disease reductio n in a single experiment; however, none repeated significant protection ach ieved in the greenhouse assay, while significant protection consistently oc curred with the fungicide chlorothalonil (Bravo). In other greenhouse trial s, neither stem injections of C. personatum nor foliar sprays of chemicals. including salicylic acid, sodium salicylate, isonicotinic acid, or benzo[1 ,2,3]thiadiazole-7-carbothioc acid S-methyl ester (Actigard), which elicit systemic acquired resistance on other crops, elicited significant disease p rotection. In contrast, foliar sprays with DL-beta -amino-n-butyric acid (B ABA), which is an elicitor of localized acquired resistance, resulted in si gnificantly less late leaf spot disease in one of two tests. Combination tr eatments of four PGPR strains with BABA in the greenhouse did not significa ntly protect peanut from late leaf spot. Field trials conducted over two gr owing seasons indicated that none of the 19 PGPR strains, applied as seed t reatments at two concentrations, significantly reduced late leaf spot disea se. The same chemical elicitors tested in the greenhouse, including BABA, d id not elicit significant disease protection. Some combinations of four PGP R and BABA significantly reduced the disease at one but not at two sample t imes. Collectively, these results suggest that late leaf spot resistance in peanut is not systemically inducible in the same manner as is resistance t o diseases in other crops by PGPR and chemical inducers.