African
Journals Online
African Plant Protection
Volume
8 Number 1 & 2 2002
ABSTRACTS
First
record of Peanut mottle virus from bambara groundnut seed in
Zimbabwe
Sibiya,
J.; Manyangarirwa, W.; Albrechtsen, S.E.
Abstract:
Peanut mottle virus (PeMoV) was detected in bambara groundnut
seeds collected from smallholder farmers in Zimbabwe. An average of
100-200 seedlings per seedlot were examined in the greenhouse and in
growth chambers for symptoms of seedborne viruses. Symptoms on leaves
were mainly a downward leaf curl and leaf narrowing and occasionally
crinkling and mosaic. Electron microscopic examination of sap from the
affected seedlings revealed flexuous, filamentous virus particles c.
700 nm long, similar to those of potyviruses. The virions in the leaf
extracts reacted strongly with antibodies to PeMoV in indirect
enzyme-linked immunosorbent assay (ELISA). The presence of PeMoV was
confirmed by inoculation of the indicator host, Phaseolus vulgaris
cv. Top Crop, which reacted by developing necrotic local lesion within
three days, and vein necrosis and yellowing within seven days of
inoculation.
Distribution and relative
importance of insect pests and diseases of maize in southern Ghana
Buadu, E.J.; Gounou, S.; Cardwell, K.F.;
Mochiah, B.; Botchey, M.; Darkwa, E.; Schulthess, F.
Abstract:
The distribution of lepidopterous maize stem borers, diseases and
physiological disorders was assessed in farmers' fields in the
southern regions of Ghana between 1996 and 1998. Maize stemborers
encountered during the surveys included, in order of importance, Eldana
saccharina, Sesamia botanephaga, Sesamia calamistis, Busseola fusca, a
Chilo sp. and the earborers Mussidia nigrivenella and Cryptophlebia
leucotreta. During the late seasons of 1996-98, significant
differences in E. saccharina and Sesamia spp. numbers as
well as stem damage were found between regions, with highest
infestations in the Ashanti region. Seven fungi (Bipolaris maydis,
Puccinia polysora, Physoderma maydis, Stenocarpella macrospora,
Curvularia sp., Marasmiellus paspali, Cercospora zeae-maydis),
two viruses (Maize streak and Maize mottle virus) and
'physiological spot' of unknown aetiology contributed to leaf area
destruction of 7-37 % depending on season and region. Significant
differences between seasons were found for S. macrospora, stem
lesions and nitrogen deficiency, with higher ratings for the second
season. Within each season, significant differences in severity of
some diseases were found between regions and the values tended to be
highest in the Volta region.
Redescription
of Pratylenchus teres Khan & Singh, 1974 (Nemata:
Pratylenchidae), with the description of a new subspecies from South
Africa, and a phylogenetic analysis of related species
Carta, L.K.; Handoo, Z.A.; Skantar, A.M.;
Van Biljon, J.; Botha, M.
Abstract:
A population of Pratylenchus teres from cotton and an unusual
new subspecies, Pratylenchus teres vandenbergae from millet and
tobacco at Rustenburg (RTB), South Africa, are described using light
and scanning electron microscopy (SEM) and according to the D3 segment
of the large-subunit (LSU) rDNA. The species is redescribed to include
the variant RTB population having a shorter stylet range, absence of a
sixth lateral line and four lip annules in about half of the
population. In both populations the SEM lip patterns are similar to
those of P. bolivianus Corbett, 1983. Their DNA sequences are
identical and moderately distant from those of P. crenatus
Loof, 1960, the originally diagnosed relative. A revised diagnosis and
a phylogenetic analysis with biogeographical implications are
presented.
Plant nematodes in South Africa. 4. Modimolle
area, Limpopo Province
Marais, M.; Swart, A.
Abstract:
During surveys of plant nematodes in the Modimolle area in the Limpopo
Province, South Africa, 182 localities were sampled. The genera Scutellonema,
Helicotylenchus, Mesocriconema and Paratrichodorus were
most frequently encountered, whereas Criconema, Criconemoides,
Hemicriconemoides, Longidorus, Meloidogyne, Ogma, Paratylenchus,
Pratylenchus, Rotylenchulus, Telotylenchus, Tylenchorhynchus and Xiphinema
were found at fewer localities. The genera Aphelenchoides,
Histotylenchus and Paralongidorus were each identified from
a single locality. Xiphinema paritaliae is herein reported as a
first record for South Africa. Various nematode-plant associations are
reported for the first time in South Africa.
Stewart's
wilt reactions of South African maize varieties inoculated with Erwinia
stewartii in field and greenhouse trials
Michener, P.M.; Pataky, J.K.
Abstract:
Eight maize hybrids available commercially in the South Africa and the
S1 generation of 25 South African varieties had resistant
or moderate reactions to Stewart's wilt based on two years of field
trials in Urbana, IL, USA. On a 1 to 9 scale, Stewart's wilt ratings
for the S1 generation of 25 South African varieties ranged
from 2.0 to 3.9 with a mean of 2.8 in 1999, and from 1.9 to 3.8 with a
mean of 3.2 in 2000. Means for the eight commercially-available
hybrids ranged from 2.1 to 3.2 in both years. Susceptible hybrids
included as controls had Stewart's wilt ratings from 4.1 to 5.0. The
levels of resistance in the maize lines from South Africa are
sufficient to minimise the effects of Stewart's wilt on yield.
Therefore, the unlikely introduction of E. stewartii to
southern Africa would have very little effect on the performance of
the South African varieties evaluated in this study. Stewart's wilt
reactions of South African varieties and US hybrids included as
controls were inconsistent in four greenhouse trials with a limited
number of replicates. Thus, the greenhouse evaluations did not provide
the same degree of accuracy as field trials. This information is
important in assessing the potential risk of introducing E.
stewartii to southern Africa, and it informs breeders and
pathologists in southern Africa about potential sources of resistance
to Stewart's wilt in germplasm that is highly adapted to the region.
Long-term effect of
preplant soil solarisation on growth of replant citrus trees in South
Africa
Cronje, C.; Le Roux, H.F.; Truter, M.;
Van Heerden, I.; Phillips, H.
Abstract:
Preplant soil solarisation was evaluated for alleviating the citrus
replant syndrome in four field experiments at three citrus estates in
South Africa. Two of the experiments involved soil that was ripped
prior to replanting, whereas the remaining two comprised interplants
in non-tilled soil and replants in soil that was disced only. No
improvement in tree growth was evident in ripped soil tarped for six
weeks. In the interplant experiment, solarisation increased canopy
volume of Valencia orange on Volckameriana lemon rootstock by 129%,
canopy mass by 203% and root mass by 141% after 30 months. In the
experiment in which soil was disced only, solarisation improved fruit
yield in the seventh year after planting by 129% in Robyn navel orange
on Rough lemon, 80% in Lane Late navel orange on Rough lemon, 145% in
Robyn on Volckameriana lemon and 259% in Lane Late on Volckameriana
lemon. However, no differences were evident between solarised and
non-solarised soil in the yield of Robyn and Lane Late grafted on the Phytophthora-tolerant
rootstock, X639 citrange. Solarisation did not have a prolonged
suppressive effect on populations of Fusarium solani, Pythium
spp. and Tylenchulus semipenetrans in ripped or non-ripped
soil, but reduced levels of Phytophthora nicotianae to zero in
non-ripped soil immediately after solarisation. In the interplant
experiment suppression of P. nicotianae lasted for less than
six months, whereas populations of the pathogen in soil that was
disced remained suppressed for 10 years under Robyn navel on X639
citrange only. Ripping per se reduced P. nicotianae in the
topsoil to undetectable levels for an extended period. This, and the
lack of response to solarisation of scions budded on X639 citrange,
suggest that the tree growth-promoting effect of solarisation was due
primarily to the initial suppression of P. nicotianae. However,
considering the lasting effect on growth and yield, the involvement of
other factors in enhanced tree performance by solarisation cannot be
excluded.
Susceptibility
of lupin cultivars to South African isolates of Colletotrichum
gloeosporioides associated with lupin anthracnose
Koch, S.H.; Ghebremariam, D.S.; Swart,
W.J.
Abstract:
Lupin anthracnose caused by Colletotrichum gloeosporioides is a
worldwide problem in the cultivation of lupins (Lupinus spp.)
and has been present in South Africa since 1993. Six local isolates of
the pathogen obtained from different localities were compared with an
isolate collected in 1995. Fourteen-day-old seedlings of 16 lupin
cultivars were spray-inoculated in the greenhouse with the seven
isolates. Disease severity was rated 14 days after inoculation. No
differences in virulence were detected among isolates. The most
susceptible selections of Lupinus albus were cultivars Kiev
Mutant, Esta, Swartland and Vladimir while Lupinus angustifolius
cultivars Wonga and Tanjil were more resistant.
Susceptibility
of pasture crops to Rhizoctonia solani and other fungi
associated with crater disease of wheat on the Springbok Flats, South
Africa
Meyer, L.; Van Dyk, K.
Abstract:
The pasture crops Anthephora pubescens, Avena sativa, Cenchrus
ciliaris, Chloris gayana, Digitaria eriantha, Festuca elatior, Lolium
multiflorum, Panicum maximum, Pennisetum glaucum, Sorghum sp. and
X Triticosecale were compared in the greenhouse with wheat (Triticum
aestivum) in crater disease (CD) soil and CD-free soil from three
wheat fields on the Springbok Flats, South Africa. All the pasture
crop species grown in CD soil were infected by the CD pathogen, Rhizoctonia
solani AG-6, and developed root nodules typical of CD, but only A.
sativa, C. gayana, P. maximum, Sorghum sp., X Triticosecale
and wheat showed significantly retarded shoot growth. Virulence of R.
solani AG-6 to all the pasture crops was verified in artificial
infestation studies with fungi isolated from pasture and wheat roots
on the Springbok Flats. Most of the pasture crops were also
susceptible to Sclerotium rolfsii, Pythium aristosporum and Pythium
arrhenomanes, whereas Fusarium nygamai, the dominant
coloniser of pasture roots on the Springbok Flats, impeded growth of
five of the crops in artificially infested soil.
Effect
of plant age, temperature and dew period on the severity of
anthracnose of cowpea caused by Colletotrichum dematium
Pakela, Y.P.; Aveling, T.A.S.; Coutinho,
T.A.
Abstract:
Three factors that influence anthracnose of cowpea caused by Colletotrichum
dematium were studied in the greenhouse, namely age of the plant
at inoculation, incubation period of the pathogen and temperature.
Cowpea seedlings were inoculated with a 5 × 105 ml-1
conidial suspension of C. dematium at three, six and nine weeks
after sowing. The inoculated seedlings were maintained in humidity
chambers (RH 95%) for 12, 24 or 36 hours and then transferred to
greenhouses at 20, 25 or 30oC, respectively. Disease
severity was rated on a 0-5 scale. Plants inoculated three weeks after
sowing were more resistant to infection than those inoculated at six
and nine weeks. There were no significant differences in disease
severity between plants maintained in humidity chambers for 12 or 24
hours. Cowpea plants were more susceptible to C. dematium at
the age of nine weeks and maintained at temperatures of 25 and 30oC
for 24-36 hours at high humidity.
Conidial
morphology and development of Alternaria cassiae from cowpea
Van den Berg, N.; Aveling, T.A.S.; Van
der Merwe, C.F.
Abstract:
Alternaria cassiae (PPRI 6393) produced a mixed population of
three distinct conidial types both in vitro and in vivo. Conidia with
aseptate, long, filiform beaks were produced more frequently on
various media except potato dextrose agar. Conidial body and beak
sizes were variable when measured in culture and on cowpea leaves.
Conidia produced on cowpea leaves in vivo were smaller in size than
those produced in culture. Conidia of A. cassiae were large,
obclavate and formed singly or in chains. Conidiophores emerged
directly through the epidermis or stomata of inoculated cowpea leaves.
Hyphae growing on the leaf surface also differentiated into
conidiophores. Conidiophores were straight or curved and enlarged
apically at the site of conidium production. Smooth, bud-like conidial
initials were produced at the apex of the conidiophores. As conidia
matured, they became elliptical to obvate and densely verrucose. Once
the mature conidium had seceded, a small pore was visible at the apex
of the conidiogenous cell.
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