Plant Pathology Fact Sheet
Late Blight on Potatoes and Tomatoes
Tom Kucharek and Pete Weingartner; Professor Extension Plant Pathologist,
Department of Plant Pathology Gainesville and Associate Professor Plant Pa-
thologist, Hastings-REC, respectively, University of Florida, 1979, Revised
Florida Cooperative Extension Service/ Institute of Food and Agricultural Sciences/ University of Florida/ Christine Waddill, Dean
Cause and Symptoms
Late blight is a fungal disease that can
devastate a tomato or potato field within two
or three weeks if it is not properly controlled.
Few diseases spread as quickly as late blight.
Thus, it is in your best interest to understand
the life cycle of this disease, recognize its symp-
toms, and know all of the control measures
available. Remember, late blight is historic as
it was partially responsible for the Irish famine
in the 1840's.
Late blight is caused by the fungus
Phytophthora infestans which thrives when cool
and wet weather occurs. Specifically, tempera-
tures between 50 and 80F. are conducive for
disease development when combined with
moisture conditions such as rain, fog, heavy
dews, or relative humidities above 90 percent.
Night temperatures in the mid-fifties with day-
time temperatures from the mid-fifties to mid-
seventies are ideal for this disease. In order for
late blight to occur, the fungus must be present.
The fungus grows by hyphae (microscopic
threads) inside infected plant tissue.
Once the fungus is in a field via diseased
tubers, transplants, etc., spores are produced
that are disseminated by wind, rain, and irriga-
tion. Temperatures at the upper range of spore
(sporangium) production (upper 60sto 80F.)
stimulate direct germination of these spores.
Temperatures in the lower range (50 to
70F.) stimulate the formation of many
swarmspores zoosporess) from within the spo-
rangia; this latter situation obviously increases
the potential for disease spread.
Where does the fungus come from? For
potatoes, seed pieces can harbor the fungus as
can cull piles and unharvested, infected tubers
from the previous year. Some studies show that
this fungus can live over in the soil, but this
has not been demonstrated in Florida. For to-
matoes, infected transplants can serve as an
original source of inoculum. It had been ob-
served prior to 1998 that seldom did a wide-
spread late blight epidemic occur on tomatoes
in the Manatee-Ruskin area unless such was oc-
curring in the Immokalee area and Dade
County. Thus, growers in the Manatee-Ruskin
area should be aware of the late blight situa-
tion further south. Similarly, growers in the
Immokalee area should be aware of the late
blight situation further south. The epidemics
that occurred in the early 1990's in Florida were
initiated by inocula in seed tubers shipped from
northern areas of the United States.
Because the fungus, P. infestans, causes
late blight on both tomatoes and potatoes, one
might assume that late blight on potatoes came
from nearby tomatoes or vice versa. This may
or may not occur depending on the strains)
present in the area. Some strains infect only to-
matoes, others infect only potatoes, others in-
fect both tomatoes and potatoes and yet other
strains are weakly parasitic on both crops. Con-
found this with infection of strains dependent
upon genes for resistance in the plant and a con-
fusing situation is created. It is your job, as a
grower, to observe your fields for symptoms.
In the Hastings area a forecasting system,
'Blitecast', issues advisories on late blight for
Symptoms may be found on all plant
parts of tomatoes or potatoes except roots. If
potato seed pieces are infected, the seedling
may have dark brown lesions on the stem which
can girdle and kill the plant and produce spores
which can infect other plants. It is a good idea
to identify the cause of seedling disorders to
be absolutely sure late blight was not the cause.
Upperside leaf symptoms begin as pale green
to brown spots with or without a purplish tinge,
(Figures 1 and 2).
Often a pale green halo is observed
around the spots as they enlarge. On the un-
derside of the leaf, a white mildew ring is
present when leaves are wet (Figure 3). These
spots merge or expand, giving a blight appear-
ance to the plants (Figure 4). Stems turn brown
when infected and later turn black. In dry
weather, infected plant parts appear dry and
shriveled (Figure 5). Potato tubers become in-
fected by spores from leaves and stems. Ini-
tially, a shallow, reddish-brown dry rot occurs
on the skin and progresses unevenly into the
tuber (Figure 6). Tomato fruit, like potato tu-
bers, will not be infected unless the foliage is
infected first. On tomato fruit, green to brown,
greasy irregular blotches occur, sometimes en-
compassing the entire fruit (Figure 7). Later a
shriveledcondition with or without a white fun-
gus growth will appear (Figure 7).
Several control measures plus observa-
tion are absolute necessities if late blight is to
be properly controlled. Tomato growers should
purchase disease-free transplants. This is not a
seed-borne disease on tomatoes. Observe your
fields thoroughly each day, especially when
cool and wet weather prevails.
Begin a fungicide spray program at the
first sign of disease, or before, if late blight is
present in your area in other fields. Ask your
county Extension agent about currently recom-
mended fungicides. Volunteer tomato or po-
tato plants should be destroyed.
Potato growers should: 1) Purchase cer-
tified, disease-free seed pieces. 2) Prior to plant-
ing, seed should be stored in a dry location. 3)
When preparing seed pieces or while planting,
examine seed pieces for tuber disorders and
destroy suspect seed. Ask your county Exten-
sion agent about getting a diagnosis of seed
piece disorders. Remember, prevention is the
key to success. 4) Destroy cull piles. 5) Destroy
volunteer potato or tomato plants. 6) Plant re-
sistant varieties. Most red-skin varieties are
susceptible. 7) Begin a spray program with fun-
gicides recommended by your county Exten-
sion agent if late blight is in your area (other
fields included) or weather conditions (see
above) are suitable for late blight development.
Forecasting systems, like that in the Hasting's
area, can help in deciding when to spray. 8) Kill
infected foilage prior to harvest to minimize
tuber infection. 9) Storage of potatoes, even for
one day, should be under dry and ventilated
conditions. 10) Discard infected tubers prior to
storage or transit.
The fungus P. infestans is capable of pro-
ducing thick walled oospores which allow the
fungus to survive in soil or old infected plant
debris. However, the presence of two mating
types, Al and A2, must be present together at
Figure 1. Late blight lesions in tomato leaflet
the same site for mating to occur. Up through
1992, only the Al mating type existed in Florida.
Beginning in 1993, both mating types have been
identified in Florida.
With the potential for the late blight fun-
gus to go through a sexual cycle in Florida, the
possibility exists that different strains of the
fungus will occur. The new strains may relate
to increased virulence, survivability, or other
characteristics. Further, the presence of
oospores in the soil provides another source of
inoculum for future epidemics. Prior to 1993,
infected seed pieces were considered as the
primary source of inoculum.
Figure 2. (Left) Late blight lesion in potato
Figure 3. (Right) Sporulation on underside
of potato leaflet.
Figure 4. Advanced late blight symptoms on
Figure 5. Advanced late blight lesion in
Figure 6. Potato tubers with late blight. ( Photo by H. Bissonnette)
i-L'-, T r- T7'"% '-'1C *L T
Figure 7. Tomato fruit with late blight.