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i4 Ft. Pierce ARC Research Report RL-83-1 April 1983
Four diseases of Siratro and other Macroptilium atropurpureum
accessions in south Florida. -
R. M. Sonodal HUME LIBRARY
ABSTRACT JUL 26U 983
An angular leaf spot of Macroptilium atropurpu um ir idyo l
fungus similar to Isariopsis griseola (Cercospora co. s
bean is prevalent in the spring and fall in south Flof-rT A yet to be
identified fungus incites leaf infections that result.in a leaf mottle
reminiscent of a mosaic-type virus during the cooler months. Two stem
lesions, one incited by Rhizoctonia solani and the other by a Pyrenochaeta
sp. are common on M. atropurpureum in south Florida.
Macroptilium atropurpureum (DC) Urb. is viny summer-growing legume
used to a limited extent for forage (5) in commercial pastures in south
Florida. Several diseases have been reported on the plant in south
Florida (5, 8, 9). Four diseases, three previously unreported, in south
Florida are discussed in this report.
Angular Leaf Spot
*Lesions with fungal structures that appear dark-gray in mass are
usually found on the lower surface of M. atropurpureum leaves in the
early spring and late fall in south Florida. The fungus is similar to
Isariopsis griseola Sacc (Cercospora columnaris Ellis & Everhart), the
causal-agent -of an angular-leaf-spot-on common bean-(3). --Angular-leaf
spot on M. atropurpureum was previously reported by the author (9) to
be incited by a Cercospora sp.. Although the range of I. griseola on
common bean is reported-to include.Florida (1), it is not listed among
the pathogens of common bean in the "Index of Plant Diseases in Florida"
(10). The pathogen is reported on common bean in the tropics and sub-
tropics (2). Isolates of the fungus on M. atropurpureum have cylin-
drical, light olive-brown conidia about 5 j wide and up to 70 .1 long.
Heavily infected leaves deep within the canopy turn yellow and drop off.
The disease occurs at the time of the slowest growth of the host, during
the cooler months, a time when availability of forage is low. The
effect of the disease on forage yield or quality has not been determined.
Fungal leaf mottle
This disease has been observed over the past several years on M.
atropurpureum during the cooler months of the year. When viewed from
the upper surface, leaf symptoms of the disease were reminiscent of a
mild viral mosaic disease. The lower surface of infected leaves had
an orange cast. There was a more pronounced orange cast in the areas
of the leaves with the most severe mottling. When observed through a
microscope, mycelia of a yet unidentified fungus were present within
Professor of Plant Pathology, University of Florida, Institute of Food
and Agricultural Sciences, Agricultural Research Center, Ft. Pierce, FL 33454.
the leaf hairs. Sporulation of the fungus was profuse on the tips of
leaf hairs. The leaf hairs were necrotic. The two-celled conidia of the
fungus were hyaline and 12 p long and 5 p wide. Attempts to culture
the fungus.have been unsuccessful. The effect of the disease on forage
yield is unknown, however, the chlorosis that it causes may have some
effect on yield during the winter months.
Rhizoctonia soil canker
Tan-colored stem cankers, up to about 0.5 cm long, occur frequently
on the lower side of trailing vines. Rhizoctonia solani Kuhn is the
organism most frequently isolated from these lesions. R. solani iso-
lates obtained from these stem cankers usually belonged to anastomosis
group 4 (AG-4) (6), however, a few of the canker isolates belonged to
AG-1. R. solani isolated from foliar blight, the most important disease
of M. atropurpureum in Florida (7), all belonged to AG-1. The effect of
these cankers on yield of forage is unknown. A Rhizoctonia sp., R.
crocorum causes death of plants in Australia (4). R. crocorum has not
been found on M. atropurpureum in Florida.
Purple stem canker
Purple-colored lesions can usually be found on mature M. atropurpureum
stems. Lesions from less than 1 mm to about 3 mm in length are scattered
over the surface of the stem. Although several fungi have been isolated
from these lesions, the organism most frequently isolated has been a
Pyrenochaeta sp. Fruiting bodies of a Pyrenochaeta sp. have been observed
in some cankers. The effect of these cankers is not known.
1. Anon. 1960. Index of Plant Disease in the United States. USDA Agr.
Handbook No. 165.
2. Cardona-Alvarez, C. and J. C. Walker. 1956. Angular leaf spot of
bean. Phytopathology 46:610-615.
3. Chupp, C. 1953. A monograph of the fungus genus Cercospora. Published
by the author, Ithaca, N.Y. 667 pp.1
4. Jones, R. M., J. L. Alcorn, M. C. Rees. 1969. Death of Siratro due
to violet root rot. Trop. Grassl. 3:137-139.
5. Kretschmer, A. E., Jr. 1972. Siratro (Phaseolus atropurpureus D. C.)-
a summer-growing perennial pasture legume for central and south
Florida. Fla. Ag. Exp. Sta. Circ. S-214. 21 pp.
6. Parmeter, J. R., Jr. R. T. Sherwood, and W. D. Platt. 1969. Anastomosis
grouping among isolates of Thanatephorus cucumeris. Phytopathology
7. Sonoda, R. M. 1980. Reduction of forage yield of Siratro by Rhizoctonia
foliar blight. Plant Disease 64:667.
8. Sonoda, R. M. 1976. Reaction of Macroptilium atropurpureum and related
species to three diseases in Florida. Trop. Grassl. 10:61-63.
9. Sonoda, R. M. 1975. Diseases of Siratro and other Macroptilium atro- -
purpureum selections in Florida. Ft. Pierce ARC Res. Rept. RL-1975-3.
10. Wehlburg, C., S. A. Alfieri, Jr., K. R. Langdon, J. W. Kimbrough. 1975.
Index of plant diseases in Florida. Bulletin 11, Division of Plant
Industry, Florida Dept. of Agriculture and Consumer Services. 285 pp.