2. Brown Rot of Potato:
Brown rot, also known as bacterial wilt, is one of the most destructive
diseases of potato.
It is the second most limiting phyto pathological factor to potato
production in Nepal.
Losses due to disease range from 10-70%.
The disease is caused by the bacterium Ralstonia solanacearum,
previously known as Pseudomonas solanacearum.
Apart from solanaceous crops like chilli, potato, tomato, and eggplant the
pathogen has been known to attack a large number of other plants
including castor, groundnut, banana and ginger.
It is very often associated with the root knot nematodes which affect
potatoes and tomatoes.
3. Symptoms:
The symptoms of bacterial wilt infection can be seen on all parts of infected
plants.
The characteristic symptoms of disease are wilting, stunting and yellowing of
the foliage followed by collapse of entire plant.
Wilting is first seen as a drooping of the tip of some of the lower leaves
similar to that caused by a temporary shortage of water.
Leaves become yellow at their bases, then the whole plant wilts and dies.
When stems are cut a brown colored ring will be visible.
When a tuber is cut in half, black or brown rings will, however, be visible. If
left for a while or squeezed, these rings will exude a thick white fluid.
A further symptom is fluid coming out of tuber eyes. This can be signified by
soil sticking to tuber eyes when crops are harvested. Serious infection causes
tubers to rot.
5. Diagnosis:
Bacterial streaming test: An easy way to diagnose this disease is to cut
the stem at the base of the plant and check for discolored vascular
tissues and pith. If present, rinse the stem and suspend it in a glass of
water. If infected, a whitish substance consisting of bacterial cells and
slime will flow from the xylem into the water within 3 to 5 minutes.
6. Disease cycle & Epidemiology:
Bacterial wilt pathogen can survive in soil (without a host for several
seasons), water, seed tubers, potato plant remnants.
The wilt bacterium is able to survive for periods up to 2-3 years in
bare fallow soils, and for longer periods in soils cropped to non-
solanaceous crops.
The disease can spread from field to field or from plant to plant
within field via infected seed, air, water, soil, farming tools, livestock
and people. Spread of bacteria by aerial means and subsequent plant
contamination through foliage is not known to occur.
However,bacteria move from roots of infected plants to roots of
nearby healthy plants, often via irrigation practices.
Soilborne organisms, such as the root-knot nematode, can cause
injury to plant roots and favor penetration of the bacterium.
High temperature (85-95°F) , soil moisture and low pH favours
disease development.
The disease spread rapidly in the warmer temperatures in storage
areas. Infected seed can also be a source of the disease in the field.
7. Management:
Control of R. solanacearum by planting resistant cultivars has been
ineffective for potato, since resistance in this host plant may vary with
location and temperature.
Certified seed free from brown rot disease should be planted.
In case cut potato tubers are being used, they should be kept at 12° C
for four days so that the cut surface hardens. The tubers can also be
treated same way as whole tubers.
The crop debris should be collected and burnt.
Control of weeds and root-knot nematode populations
12kg/ha bleaching powder at planting time in furrow.
Summer ploughing after harvest.
Practice good crop rotation. A one in four (or more) rotation should
be practiced.
