Management of purple blotch of onion, garlic and shallot using antagonists and systematic acquired resistance

Onion and shallot farms surveyed in Bongabon, Gabaldon, Muñoz and San Jose City, Nueva Ecija [Philippines] had low incidence of purple blotch caused by Alternaria porri during the 2002 planting while leaf anthracnose caused by Colletotrichum sp. was severe. The incidence of purple blotch was also low in garlic farms in Batac, Ilocos Norte [Philippines]. The Colletotrichum isolate was highly pathogenic to both onion and shallot while the Alternaria isolate was weakly pathogenic. A total of 80 probable bacterial antagonists were isolated from leaves and roots of garlic, onion and shallot. Fifteen isolates inhibited mycelial growth of the Alternaria isolate based on dual culture test. Five isolates have strong inhibitory effect on conidial germination of the pathogen. Those that germinated have swollen germ tube. Since the Alternaria isolates did not produce typical purple blotch symptoms in onion, garlic and shallot in subsequent tests, the potential antagonists were evaluated against the Colletotrichum isolate. In the first trial, out of seven potential antagonists were evaluated under greenhouse conditions using the Tanduyong variety, four isolates produced relatively fewer leaf lesions compared with untreated plants four days after inoculation. In another trial, 8 out of 18 antagonists produced fewer anthracnose lesion on leaves of shallot compared with plants sprayed only with water (control). The bacterial antagonists QST, EPSP, BC1 and CS2HT were more effective in reducing anthracnose severity of shallot when grown in liquid medium like potato dextrose peptone broth than in solid medium like PDPA. The compounds tested (acetylsalicylate, isonicotinic acid, K2HPO4,oxalic acid and salicylic acid) as possible inducers of resistance against Allium anthracnose did not provide consistent protection against the disease. Plants sprayed with 20 mM oxalic acid had relatively lower disease severity compared with untreated plants. None of the plant extracts(fresh or compost extract) increased the resistance of the test plants against onion anthracnose. In field test, the antagonists B2HT3 and QST and the chemical inducers oxalic acid and benzothiadiazol(BTH) or acibenzola-s-methyl were effective in reducing anthracnose severity of shallot and were comparable to the effect of 150 ppm difenoconazole.