A new leaf spot disease of Chinese amaranth
(Amaranth mangotanus L.) caused by Rhizoctonia solani was frequently observed at
the so-called organic farms in Taiwan during the summer season. The hyphae of
RSA-03 and RSA-09 isolates obtained from Chinese amaranth leaf spot were able to
anastomose in high frequency (> 70%) with R. solani AG 2-2 IIIB (ATCC
76124), but in low frequency (< 5%) with R. solani AG 2-1 (ATCC 76168),
AG 2-2 IV (ATCC 76125), AG 2-3 (R6) and AG BI (ATCC 76132). When five isolates
from Chinese amaranth leaf spot were respectively cultured in liquid glucose
asparagine (GA) medium with or without thiamine-HCl, they were auxtrophic for
thiamine-HCl and more closely resembled the ATCC 76124 isolate of R. solani AG
2-2 IIIB. The optimum temperature for mycelial growth of RSA-09 isolate from
Chinese amaranth leaf spot was similar to one of R. solani AG 2-2 IIIB (ATCC
76124). Inoculation tests revealed that both RSA-03 and RSA-09 from Chinese
amaranth leaf spot and R. solani AG 2-2 IIIB (ATCC 76124) were pathogenic to
Chinese amaranth. Based on the anastomosis, thiamine-HCl requirement, growth
temperature, and pathogenicity tests, the isolates from Chinese amaranth leaf
spot were recommended as R. solani AG 2-2 IIIB. According to Koch’s postulates
tests, it was proved that the Chinese amaranth leaf spot was caused by the
basidiospores of T. cucumeris, the telemorph of R. solani AG 2-2 IIIB. The
basidiospores (105 spore/ml) of the pathogen were sprayed to leaf surface of
Chinese amaranth. The inoculated plants were put in the moist chamber at 28℃.
Primary lesions appeared as small, circular water-soaked spots on leaves of
Chinese amaranth six days after inoculation. After additional incubation,
claw-like lesions growing out from the primary lesions expanded into the leaves
tissues and caused secondary lesions with large-sized irregular necrotic spots.
When Chinese amaranth leaves were inoculated, basidiospores of the pathogen
germinated and penetrated into the epidermal cell walls nine hours after
inoculation. Mass mycelia were formed 18hrs after inoculation, then development
of mass mycelia into stroma-like structure 21hrs after inoculation. In the
study, Naito’s soil-over-culture method for production of hymenia was modified.
It was found that the peat moss and soil-over culture method (PSC method) was
much more effective in producing hymenia of T. cucumeris RSA-03 and RSA-09. The
procedures of PSC method were as follows: (1) to inoculate the fungus onto
potato- yeast extract-dextrose agar plate in a 9-cm petri dish, (2) to incubate
at 28℃for 4 days until the fungal colony covered the agar plate surface, (3) the
agar plate surface was covered with 90ml soil [included 40% (v/v) BVB No. 4 peat
moss and maintained the soil moisture at 40 ~ 50% (v/v)], (4) experiments were
kept in moist chamber. After 4-day-incubation hymenial formation was observed.
The PSC method was suitable for hymenial formation of the pathogen and able to
markedly produce 3-4 fold hymenial amount compared to Naito’s soil-over-culture
method. The factors affecting hymenial formation of the pathogen included
temperature, humidity, light, aeration, and culture substrate. The temperatures
were favorable for R. solani RSA-03 and RSA-09 hymenial formation at 24 ~ 28℃and
the covered soil was at pH 5 ~ 7. The amendments of covered soil with various
organic and inorganic materials, antagonists, and fungicides did significantly
influence the hymenia formation of T. cucumeris RSA-03 and RSA-09. The hymenia
of the fungus were completely inhibited in the covered soils amended with
1﹪(v/v) fish meal, tea seed pomace and chinaberry meal. Amendments of covered
soil with Stretomyces padanus PMS-702, S. sioyaensis PMS-502, S. saraceticus
SS-31 and S. misionensis PMS 101 also inhibited hymenial formation, but Bacillus
pumilus PMB-102, B. thermoglucosidasius PMB-101 and B. subtilis BS-001 did not.
The fungicides, mancozeb, benomyl, carbendazim, flutolanil, PCNB, iprodione and
pencycuron were significantly effective in inhibiting the hymenial
formation.
Yangzhou pioneer chemical CO.,LTD.
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