There
are three types of blight in tomatoes. Septoria
leaf spot, Early Blight & Late Blight. This
article refers to Late Blight.
Also known as Potato
Blight, tomato blight is highly contagious
disease caused by an oomycete (or water
mould) known as Phytophthora infestans.
It affects potatoes, tomatoes and some
other members of the Solanaceae. Late
blight was responsible for the Irish Potato
Famine in the 1800's which killed 1 million
people. It can devastate commercial & home crops
in a matter of days, thriving in
wet, humid conditions.
There are two types of
tomato blight, A1 & A2, each type is able to
asexually reproduce, however A1 & A2 can also
sexually reproduce with each other, resulting in
the exchange of genes & leading to resistant
strains.
Infection occurs when
spores land on foliage. These are spread via
wind &/or rain splash. The spores penetrate the
foliage, growing into a dark lesion, which in
turn contains thousands of new spores & spreads
to new plants.
Signs of tomato blight:
Tomato blight affects the
leaves, fruit & tubers of both tomato & potato
plants. It tends to occur later in the growing
season than the other two types of blight,
around August in the Northern Hemisphere.
Common signs include;
-
Rapidly spreading,
irregularly shaped lesions on the leaves,
especially after periods of wet weather.
These may appear water soaked in wet
weather, in dry weather the centre will dry
out. Lesions spread to the stems. The lower
leaves are usually affected first.
-
During high humidity, a
white/fuzzy mildew may be observed on the
underside of the leaf.
-
Lesions appear on the
fruit as dark, greasy spots, secondary
organisms can then invade the fruit, leading
to soft-rot.
-
Eventually the entire
plant will wither & die. This disease
strikes fast & can destroy a crop within
days.
Preventing tomato
blight:
-
Fungicides can be used
to prevent blight.
-
Crop rotation. Plant
tomatoes & potatoes in different garden beds
every year.
-
Water carefully. Don't
spray tomato plants as this can spread the
fungus. Water only at the base of the plant,
preferably early in the day.
-
Frequently monitor the
leaves & fruit for signs of blight. If you
find some, remove the affected plant. You
may be able to save the fruit by picking.
The plant should be
placed in a plastic bag & carefully disposed
of to prevent the disease spreading to neighbouring gardens or farms.
Do not put infected plants or fruit in
your compost bin.
-
Mulch. This not only
reduces the need to water as often but also
helps prevent splash up from rain &
watering.
-
Ensure there is plenty
of space between tomato plants to allow for
good air flow.
-
Be careful when bringing tomato
plants/cuttings etc., into your garden. Make
sure you choose healthy looking plants with
no sign of disease.
-
Try planting blight
resistant varieties.
-
While not a preventive
measure, plants should be checked every day
or so for signs of late blight.
How to treat tomato
blight:
Fungicides can be used to
prevent blight, but once your plant(s) has
become affected then it's generally too late to
treat.
Fruit may be removed from
the plant, the plant should then be carefully
destroyed. Infected plants should be placed in a
plastic bag & placed in the bin. Do not put
infected plants in the compost.
Fruit may be edible, leave it for a
few days to see if it develops lesions. If it
doesn't, then it is fine to eat. Discard any
fruit that develops signs of tomato blight.
Acknowledgements:
Thanks to Craig for the use
of his tomato photos.
Also see:
Bulb planting guide,
Caring for your fruit trees,
Companion Planting,
Growing fruit & vegetables in containers,
Growing garlic,
Growing potatoes,
Home composting,
Mulching basics,
Powdery mildew,
Saving tomato seeds,
Snail & slug Control,
Whitefly
