Abstract
Secondary transmission of Acidovorax avenae subsp. citrulli (Aac) from infested watermelon seedlings to healthy nursery plants at the nursery was studied. Infested watermelon seeds were sowed in four single 3-cm square plastic cells at the center of 80-cell trays that had been sowed with healthy seed and irrigated overhead 3 times/day. Aac from infested seedlings and from diseased seedlings spread to healthy seedlings around the point source of Aac, and a numerous seedlings became infested by Aac. When scions were cut from healthy watermelon seedlings with knives that were infested from cutting diseased seedlings or from sap of watermelon seedlings inoculated with cultured Aac (106cfu/ml), all grafted nursery plants became diseased. When scions were prepared with knives used to cut infested seedlings without primary symptoms, 67.5% of grafted nursery plants became diseased. Aac was also transmitted from infested scions to healthy ones from wet newspapers and tap water used to maintain humidity around scion. Secondary transmissions thus occur through overhead irrigation and grafting techniques. Symptomless Aac carrier nursery plants were produced from a small amount of infested watermelon seeds.
