Phytophthora infestans responses to stinging nettle extract, phosphoric acid and fungicides combination, in Kenya

Late blight is a significant disease of solanaceous
crops
worldwide. Knowledge on sensitivity of late blight
pathogen (
Phytopthora infestans
) to metalaxyl and different
fungicides is important for effective management of
late
blight.
In vitro
studies were carried out to evaluate the
sensitivity of
P. infestans
to different levels of Metalaxyl
Ridomil, Dithane M45, phosphate and stinging nettle
.
Sensitivity to metalaxyl and other different fungic
ides was
determined by growing
P. infestans
isolates on 15% V8
medium amended with 0, 5 and 100 ppm metalaxyl and
20ppm of Ridomil, 25 ppm of Dithane M45, 20ppm of
Phosphate and 1ml of stinging nettle extract in 1ml
of
water. The pathogen isolates were from the leaves c
ollected
from different potato growing areas of Kenya. Sensi
tivity
was determined by measuring the colony diameter at
14
and 21 days, counting the numbers of spores and
determining the weight of the mycelium.The result o
f the
experiment indicated that there was intermediate
resistance of
P. infestans
to
metalaxyl in all the region as the
growth was recorded as 57.8% in 5 ppm and 19.8 in 1
00
ppm in Tigoni station and 58.3 in 5 ppm and 19.7 in
100
ppm in Njambini while in Meru region the growth was
52.9
in 5 ppm and 25.9 in 100 ppm metalaxyl relative to
the
control. Njambini and Tigoni regions did not differ
significantly in term of resistance. However both r
egions
differed significantly with Meru region in terms of
weight of
mycelium and number of spores, Meru location having
the
highest number of spores and weight of mycelium
compared to both Tigoni and Njambini locations. All
the
fungicides tested differed significantly in their e
ffect on
pathogen colony diameter, number of spores and weig
ht of
mycelium. The reduction of colony diameters of isol
ates
collected from Tigoni of 41.7 and 80.3 in 5 and 100
ppm
metalaxyl did not differ significantly with that fr
om
Njambini significantly of 42.2 mm and 80.2 mm in 5
and
100 ppm respectively. The pathogen was more sensiti
ve to
phosphoric acid and least sensitive to stinging net
tle
extract

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