Biology Reference
In-Depth Information
Fig. 5
Late Blight of Potatoes. Sporangiophores of
Phytophthora infestans
emerging from leaf, bearing sporangia,
sometimes called conidia, which germinate by zoospores
Tubers with only a thin covering of soil may be
infected by swarmspores washing down onto
them from blighted leaves overhead; they are
also infected during digging if it is done in
moist weather while tops are still green.
Swarmspores remain viable in the soil several
weeks while awaiting favorable conditions.
Oospores, the sexual spores, are apparently not
required in the life cycle for they are not found
with potatoes grown in the field. They have been
produced in culture.
Weather Relations
This is a disease entirely
dependent on weather conditions. Temperature
and moisture conditions are right for an epiphy-
totic about 2 years out of 5. Zoospores are pro-
duced only in cool weather, 60
F and under, but
they invade leaves most rapidly at higher temper-
atures. Because they are swimming spores, rain is
required. A cool, wet July is usually followed by
blight in August and September.
Control
Some varieties, such as Kennebec, Essex,
Pungo, and Cherokee, are resistant to the common
strain of the fungus but not to some of the newer
strains. Treat potato dumps and cull piles with
a weed spray to control sprouts. Delay digging crop
until 2 weeks after tops die, or else kill the tops with
a weed killer to prevent infection at early digging.
Late Blight of Tomato
Although there are potato and tomato strains of
Phytophthora infestans
, each is capable of
infecting the other host. Ordinarily blight starts
with potatoes in midsummer; when the fungus
moves over to tomatoes, it has to go through
several cycles to build up a strain virulent enough
to produce general blighting, and by that time the
tomato season is nearly over. Now we know that
it is possible for the tomato strain to winter in
potato tubers and be ready to inflict damage on
tomatoes with the first crop of zoospores pro-
duced on potato sprouts. Conversely, tomato
seedlings brought up from the South and planted
near potato fields can start an epiphytotic of late
blight on potatoes.
The 1946 tomato blight saga - the one that
awakened eastern gardeners to the fact that plant
disease could be as important to home gardeners
as to farmers - started in Florida late in Novem-
ber 1945. By January the disease was extremely
destructive in tomato seedbeds, and it continued
so intermittently whenever temperatures ranged
from 60
to 70
F and relative humidity was
nearly 100 % for more than 15 h. Evidence