| Symptoms |
The disease is first seen as damp areas on the
lower leaves of plants that may fuse to form a large brown area
of dead or damaged tissue. The pathogen grows out from the leaves
on the lower surfaces of the leaves where it is seen as a white
downy mass. The disease is at most aggressive under damp conditions
and will rapidly kill all the aerial parts of a plant. A cycle of
infection to sporulation can take as little as four days. If there
is a dry period, the disease pauses but will resume when the weather
turns damp again.
Infected potato tubers show surface damage only, but the damage
may allow other microorganisms to enter the tuber and destroy it.
The rot can be so severe that entire fields may smell of rotting
vegetation. |
| Prevention and Treatment |
The first stage in control of the disease is prevention
by good field husbandry. Disease-free seed potatoes should be used
for planting and potato waste should be burned or treated with herbicides
as should volunteer plants. Disease-resistant varieties should be
used when possible and farmers should keep abreast of news of outbreaks
to select varieties and treatment. The pathogen is at its most virulent
in areas with cool, damp climates or where the soil has become overwatered
or over-irrigated so good management of soil water content becomes
important when an outbreak is reported.
The infection can be treated by repeated spraying with fungicides
including:
Repeated spraying may be necessary and even resistant varieties
of plant may need more than one application. Ground spraying is
more effective and economical, but aerial spraying may be required
for some some cultural practices.
The disease can spread widely and rapidly, effective communication
and monitoring of outbreaks through local or national authorities
is essential for control. |
| Other Comments |
This is another organism that can be used as a paradigm
of the social dislocations that biological weapons are capable of.
An outbreak of P. infestans swept across Europe in the 1840's,
destroying potato crops after the potato had established itself
as the primary starch food. P. infestans is native to South
and Central America and was unknown in Europe so it broke out unhindered
by human knowledge. The outbreak was most devastating in Ireland.
There it led to the death of 1 million from starvation and forced
the migration of 1.5 million poor and landless Irish to the Americas
where they had a massive impact on the growth of the nation.
P. infestans is only the most important member of an order
of quite thuggish plant pathogens, the Peronosporales. Unlike
other pathogens, especially fungal pathogens, the Peronosporales
do not seem to have their pathogenicity moderated by the complex
interaction of virulence and avirulence factors that create a balance
between plants and fungal or bacterial pathogens. Outbreaks of these
agents are often extremely destructive. All members of the order
are plant pathogenic and the genera of the order include:
| Albugo |
Relatively
mild pathogens causing white rusts of cruciferous plants (cabbage,
cauliflower, broccoli, rape.) |
| Bremia |
Destructive
pathogens causing downy mildews of dicotyledonous plants (crop
plants other than grasses and cereals such as lettuce and
alfalfa.) |
| Peronosclerospora |
Destructive
pathogens of monocotyledonous plants - mainly the grasses
and cereals - causing downy mildews of corn and sugarcane
amongst others. |
| Peronospora |
Similar
to Bremia. |
| Plasmopara |
Similar
to Bremia |
| Phytophthora |
Causing
rot of lower stems and roots of dicotyledonous plants, of
which potato blight is the most serious, and rot of soft fruits.
They attack a number of important vegetable and fruit crops. |
| Pseudoperonospora |
Similar
to Bremia. |
| Pythium |
Causes
damping off of seedlings. It is a common disease of turf grasses
and also causes root and soft fruit rots. |
| Sclerophthora |
Similar
to Peronosclerospora. |
| Sclerospora |
Similar
to Peronosclerospora. |
The Peronosporales were considered to be fungi for many years but
were recently moved to the newly defined kingdom of the Chromista
that includes the algae and seaweeds. These organisms resemble fungi
in their patterns of growth, breeding systems and many other aspects
of their biology. P. infestans has been said to be "trying
real hard to be a fungus." |