BIOLOGY AND EPIDEMIOLOGY OF SPIIACELOMA sp. The PATHOGEN OF COWPEA SCAB.
BIOLOGY AND EPIDEMIOLOGY OF SPIIACELOMA sp. The PATHOGEN OF COWPEA SCAB.
No Thumbnail Available
Date
1996-12
Authors
MUNGO, CATHERINE MOUKOU
Journal Title
Journal ISSN
Volume Title
Publisher
Abstract
Transverse sections of scab infected cowpea pods revealed the presence of conidia
and conidiophores in the acervuli of Sphaceloma sp. on the pods and peduncles.
The pathogen colonized the epidermis in which it produced stromatic tissue from
which the conidiophores and conidia developed.
Sphaceloma sp. was isolated either by plating out intact young pod or leaf
lesions or by grinding these (lesions) to a paste which was subsequently dispersed
on media containing cowpea extracts or on potato dextrose agar (PDA) amended
with streptomycin sulphate and rose bengal. Cowpea pod agar (CpdA) and PDA
amended with streptomycin sulphate (1.5 g/1) and rose bengal (0.0025 g/1 of agar)
were the best media for isolation of Sphaceloma sp., when intact lesions were
plated on the agar surface. The dispersal of a paste of ground lesions on agar
plates was less tedious and less time consuming than plating individual lesions.
Scab symptoms were induced earlier on younger cowpea plants than on
older plants after plants were sprayed to run-off with a conidial suspension of
Sphaceloma sp. The incubation periods for the leaf, peduncle and floral cushion
were 5, 9 and 8 days, respectively, regardless of the age of plants at inoculation.
However, incubation period varied from 7-10 days on the stem, depending on the
age of the plant at the time of inoculation.
In agar culture, the fungus grows vertically, producing very little or no
radial growth. Colonies readily come off agar plates when picked with forceps;
consequently the mycelial mass colonies of Sphaceloma sp. was used as measure
of fungal growth. These colonies, each about 2 mm in diameter and weighing
approximately 0.00lg, were incubated at temperatures of 15, 20, 26, 30, and 35
°C for 3 to 15 days on corn meal agar plus rose bengal (CMAR), malt agar plus
rose bengal (MAR), and potato dextrose agar plus rose bengal (PDAR). The
fungus grew at 15-26°C on all three media, but no growth occurred at 30°C.
Colony weight loss was recorded on MAR and PDAR at 35°C. Among the three
media, PDAR was the best for growth of Sphaceloma sp., with maximum growth
occurring at 26° C. On CMAR and MAR the fungus flattened out somewhat
instead of growing vertically as it does on PDAR. By the 15th day at 26°C, a
gelatinous matrix coated MAR colonies. Streaking this matrix on PDAR and
MAR and incubating the plates at 26°C produced many colonies of the fungus.
Attempts to isolate the fungus from infected cowpea seeds were unsuccessful.
In view of this, an indirect method was used to determine if scab is seed
transmitted in cowpea. In the first year of the study, the incidence of scab on
seedlings grown from seeds produced in a region where little cowpea is grown
and scab does not occur was significantly lower than on those from seeds
produced in a scab-infected field, despite the fact that seedlings were probably
affected by inoculum from an adjacent, older cowpea field. In the first and
second year, seed treatment with benomyl drastically reduced incidence of scab in
seedlings grown from seeds produced in infected fields. The data strongly
suggest seed-transmission of cowpea scab.
The time taken for scab symptoms to appear on all the fields with different
cowpea rotations, irrespective of mulching treatment, was 25-27 days in 1993 and
1994. In both years the incidence of scab in fields last planted with cowpea a year
before the trial was the highest compared to the fields last planted with cowpea 2
or 3 years before the trial. Polyethylene mulching on cowpea plots reduced rain
splashing of soil onto healthy plants and probably reduced inoculum levels of the
pathogen, resulting in low disease incidence for the mulched plots. More than
75% of the first symptoms of scab in cowpea seedlings under natural inoculation
occurred on the epicotyl. No scab symptoms were observed on the primary
leaves. The evidence suggests that the pathogen is soil-borne either in infected
trash or as perennating structures (probably chlamydospores) in the soil.
Disease incidences and apparent infection rates in both 1992 and 1993 were
lower in later plantings (P= 0.05). In 1992 there were higher disease incidences
on all the affected plant parts than in 1993.
In 1992, the grain yields obtained in the early sown crop of the variety
susceptible to scab (IT 85F-1517) were the lowest (306.0 kg/ha) though there
were no significant differences between these yields and those of the last planting
date. In 1993, the late sown crop of IT 85F-1517 had a lower grain yield (340.0
kg/ha) which was not significantly different (P= 0.05) from that of the earliest
sown crop (346.0 kg/ha). The second planting date yielded the highest grain yield
(1390.0 kg/ha). The earliest sown crop of the cowpea variety resistant to scab
IT 83S-720-2 in 1992 and 1993 had higher grain yields (2220.0 and 2580.0 kg/ha
respectively) which were not significantly different from the grain yields
obtained in the second planting dates (2067.0 and 2050.0 kg/ha). The last sown
crops had the lowest yields (1670.0 and 685.0 kg/ha in 1992 and 1993,
respectively).
Weekly applications of benomyl or carbendazim + maneb reduced the
development of scab symptoms on peduncles and gave good control of the disease
on flowering cushions and pods, compared to mancozeb and untreated control
plants treatments. Regardless of the plant part on which disease incidence was
recorded, the areas under the disease progress curves were almost zero when
benomyl or carbendazim + maneb were used to control scab. Mancozeb treated
plants had significantly higher disease severity values on all aerial plant parts in
comparison with plants treated with benomyl or carbendazim + maneb.
However, the disease severities in plants treated with mancozeb were similar to
those in the untreated (check) plants.
Description
A THESIS SUBMITTED TO THE POSTGRADUATE SCHOOL
AHMADU BELLO UNIVERSITY, ZARIA
IN PARTIAL FULFILMENT OF THE REQUIREMENTS
FOR THE DEGREE OF DOCTOR OF PHILOSOPHY
IN PLANT PATHOLOGY
DEPARTMENT OF CROP PROTECTION
FACULTY OF AGRICULTURE
AHMADU BELLO UNIVERSITY
ZARIA, NIGERIA
DECEMBER, 1996
Keywords
BIOLOGY,, EPIDEMIOLOGY,, SPIIACELOMA,, PATHOGEN,, COWPEA SCAB