Efficacy of ascorbic acid, citric acid and Thevetia sp. extract against Agrobacterium tumefaciens and Meloidogyne incognita infecting guava

The efficiency of some organic acids (ascorbic and citric acids) and methanolic extract of Thevetia sp. were investigated against crown gall bacterium (Agrobacterium tumefaciens) and the root-knot nematode (Meloidogyne incognita) under in vitro and pot trails. Two isolates (Ag1 and Ag2) of crown gall bacteria were used in this study. The isolate Ag2 was identified molecularly by PCR using universal primers for 16 S rRNA. The PCR products' DNA sequence, BLAST analysis and Genbank data revealed that the Ag2 isolate belonged to A. tumefaciens. According to the phylogenetic tree based on the DNA nucleotide sequences of the 16 S rRNA gene, the Egyptian isolate is closely similar to the Indian isolate (PP218089). The analysis of Thevetia sp. methanolic extract exhibited that 35 phytochemical components were identified by using GC-MS. Under in vitro conditions, ascorbic acid at 6.4% was the most effective against A. tumefaciens (Ag2 isolate) recording inhibition zone of 38.3 mm with disc diffusion method and 100% inhibition with MIC method at 1.00 g/l. However, Thevetia sp. extract was the most effective against 2nd stage juveniles of M. incognita after 48 h of exposure with values of 0.478 g/l (LC₅₀) and 3.185 g/l (LC₉₀). In pots trial, ascorbic acid 2 (14.8 g/l) completely suppressed both the number and weight of galls (tumors) in guava seedlings subjected to either single or dual infections of A. tumefaciens and M. incognita. Thevetia sp. 2 (6.4 g/l) and ascorbic acid 2 (14.8 g/l) significantly reduced root gall formation and soil populations by 63.40 and 70.27% under single infection of M. incognita and 81.67 and 76.57%, under dual infection of A. tumefaciens + M. incognita, respectively. Furthermore, these treatments markedly enhanced the growth performance of guava seedlings, as evidenced by increased shoot height and root length and significantly elevated the levels of total phenols and total soluble proteins under both single or dual infection scenarios.