Effectiveness of Host Resistance Inducers and Humic Acid for Fire Blight Control and Shoot Growth On Apple Cultivars with M9 Rootstock

Fire blight, caused by Erwinia amylovora, is the most devastating bacterial disease of apples with M9 rootstocks and often results in the death of the tree. The resistance-inducing compounds prohexadione-Ca (PC), harpin protein (Hp) and benzothiadiazole (acibenzolar-S-methyl) (BTH), the fertilizer humic acid (HA), the bactericides streptomycin and copper, and combinations of copper with chemicals were evaluated for their ability to control fire blight and shoot growth on apple cultivars, Gala, Red Elstar, Jonagold and Pinova, with M9 rootstock in two years. PC was applied at a rate of 125 mg L^-1 at two shoot lengths (6-12 cm and 15-20 cm), while BTH (135 mg L^-1) and harpin (50 mg L^-1) were applied when the shoots measured between 15-20 cm, and again at 30-35 cm. HA (200 mg L^-1), copper and streptomycin were applied at three phase of shoot lengths. In bioassay tests, bacterial populations were determined in plant tissues at 5^th, 10^th and 15^th days. Disease severity and effectiveness of the applications were calculated using the following formulas: Disease severity (%) = (a/b) x 100; where a is the length of the blighted part of the shoot (cm), and b is the whole length of the shoot (cm) and Efficacy (%) = [(DIck - DItr) / DIck] × 100 where DIck is mean disease severity in water sprayed control plants and DItr is mean disease severity in treated plants. When the apple varieties and years were taken into consideration, PC alone (44% - 70%) and in combination with copper compound (48% - 63%) were the most effective treatment during both years on disease control, following by the streptomycin (76% - 95%)(p ˂ 0.05). PC reduced both shoot length and shoot blight on the apple cultivars. The cultivar Red Elstar showed the lowest disease severity than the other cultivars and followed Jonagold, Pinova and Gala, respectively. HA applications were unsuccessful in controlling fire blight on all of the apple cultivars. Streptomycin treatment yielded the lowest rate of bacterial population, and it was followed by PC, Hp, BTH, copper and HA respectively. The use of resistance-inducing substances during the early phase of shoot growth may offer a means of managing the shoot blight phase of fire blight disease on apple cultivars with M9 rootstock.