Leaf spot similar to bacterial spot or bacterial shoot blight disease of tea were observed on tea leaves after a typhoon in Kagoshima Prefecture in 2007 and 2008; however, the causal agents of these bacterial diseases have not been isolated. Instead, four bacterial species that can induce a hypersensitive response (HR) symptom associated with the generation of hydrogen peroxide after infiltration of bacterial suspensions into tea leaf tissues were isolated from the leaf spot. Analyses of bacterial characteristics and the 16S rRNA gene sequences indicated that two dominant species among the four were most closely related to Herbaspirillum huttiense and Acidovorax avenae, respectively. These bacterial species did not cause lesions after needle-prick or spray inoculations under wet field conditions; however, spraying inoculum on a tea field under heavy storm conditions reproduced the same symptom within 1 to 2 d. Our data indicates that macroscopic HR symptoms can be caused by avirulent bacteria after heavy storms such as typhoons.
Lisianthus (Eustoma grandiflorum) necrotic ring spot disease caused by Iris yellow spot virus (IYSV) is one of the most devastating lisianthus diseases. We tested several insecticides under both experimental and field condition to determine the length of the their residual activity in killing the thrips vector (Thrips tabaci) and their ability to inhibit IYSV transmission. The survival rate of T. tabaci was significantly low, and their ability to transmit IYSV was greatly inhibited after treatment with either acephate or spinosad under the experimental conditions. The insecticidal activity of acephate remained for at least 28 days, the longest duration of all examined insecticides. The number of thrips on lisianthus plants treated with acephate remained low until 17 days after treatment (DAT), while thrips density had increased by 10 DAT when thiamethoxam. These results indicated that acephate most effective controls the lisianthus necrotic ring spot disease.
The mechanism underlying suppression of apple violet root rot caused by Helicobasidium mompa was studied using a fertilizer paste (product name: TP, Taki Chemical, Hyogo) in kuroboku soil (soil texture: L). In pot experiments, mycelial growth in soil treated with urea (UR), the major ingredient of TP, was strongly suppressed, but treatment with H3PO4 or KCl was not effective. Mycelial growth was not suppressed in potato-dextrose broth containing 10 mg/mL of urea or nitrates (NaNO3, KNO3), but was strongly suppressed by 1 mg/mL of ammonium carbonate (AC) and 0.1 mg/mL of nitrites (NaNO2, KNO2). In pot experiments, amendments with TP, UR or AC inhibited mycelial growth for at least 29 d. During this time, the level of NH4-N and NO2-N was higher than the threshold to suppress mycelial growth in culture. The soil microbial community changed with treatments; the species of fungi decreased, but bacteria increased. The suppressive effect of soil treatments was barely detectable 62 d after treatment, when both NH4-N and NO2-N decreased, and populations of fungi and bacteria became equivalent to that in the untreated control soil. Disease severity on apple rootstocks (Malus prunifolia var. ringo) in pots treated with UR, AC or nitrites was low as in the case treated with TP, while nitrates failed to suppress disease. These results suggest that AC and NO2-N induced by UR are effective for suppressing violet root rot of apple. In addition, the suppression mechanism may be ascribed to biological factors.
Japanese pear trees became infected with Glomerella cingulata when they were planted in plots covered with diseased leaves but not in plots that were free of fallen leaves. The fungus was isolated more frequently during the winter from buds of pear shoots sampled from damaged orchards than from those of pear shoots sampled from orchards without anthracnose. Dormant buds on diseased shoots incubated under moist conditions produced salmon-pink conidial masses. These results suggest that fallen leaves and buds can be primary inoculum sources of the fungus.