Since 1975, a new root rot disease of dasheen has been occurring at the various nursery beds in northern Chiba Prefecture. The disease was severe in high humidity and soil temperature. The infected plant with decayed roots was stunted, but its tubers remained relatively healthy. Although Pythium myriotylum and Rhizoctonia spp. were isolated from the infected roots, only Pythium myriotylum was pathogenic.
A selective medium for Corynebacterium michiganense pv. michiganense (SMCMM) was developed. The composition of the medium was as follows: peptone 5g, yeast extract 3g, K2HPO4 2g, KH2PO4 0.5g, MgSO4⋅7H2O 0.25g, glycerol 20g, LiCl 5g, K2Cr3O7 80mg, NaN3 2mg, nalidixic acid 20mg, cycloheximide 40mg, 70% tetrachloroisophthalonitrate hydrate 3mg, agar 15g in one liter of distilled water. Recovery of C.m. pv. michiganense on SMCMM from the pure culture was higher than on D2 medium by Kado et al. and CNS medium by Gross et al. C.m. pv. michiganense grew well on SMCMM and developed to the characteristic fresh yellow colonies. The colony forming efficiency of the general microorganisms on the tomato leaves and soil were very low compared with nutrient agar medium.
Two hundreds and fourteen isolates of Phytophthora nicotianae var. parasitica collected from rotted roots of strawberry seedlings on nursery beds in Saga prefecture were tested on the mating types, A1 or A2, by formation of sexual organs. Among them, 54 isolates were identified as type A1 and the others were as type A2. From the result of the mating type test, it was found that all isolates were heterothallic. The isolates from Saga city·Saga-gun, Kanzaki-gun and Kishima-gun were separated into type A1 and A2, whereas those from Karatsu city, Ogi-gun and Tosu city·Miyaki-gun were either type A1 or A2.