The Online Gardener's Handbook
Chapter 4: Vegetables
Table of Contents
- Colorado Potato Beetles and Blister Beetles
- Common Scab
- Early Blight
- Hollow Heart
- Late Blight
- Learn More
Always plant certified seed potatoes. Saving potatoes from previous
crops can introduce viruses or other diseases into your garden,
which remain in the soil and infect plants for years. Do not use
table potatoes purchased from the supermarket. They are often treated
to prevent sprouting and will not develop into good plants.
In this chapter, a description of various potato pests will be
provided along with suggested management options. These management
options will not include the use of pesticides. Some biopesticides
and certain reduced risk pesticides are still available to the homeowner
for controlling weeds and pests in lawns and gardens. For more information,
refer to Chapter
2 of this handbook and the Ministry
of the Environment's website. For suggestions on managing specific
weeds and pests, consult local horticulturalists, Master
Gardeners or your local garden supply centre.
Anthracnose fungi persist in soil and crop residue and start below
the soil surface on potato, causing brown dead areas along, and
sometimes completely girdling, the stem. The tubers are infected
causing black dots oin the outer skin of potato and inside hollow
Rotate crops. Clean up plant residue when digging potatoes to prevent
overwintering of spores.
Colorado Potato Beetles and Blister Beetles
For information and management options, refer to the Colorado Potato
Beetle section in Insects and Diseases
Affecting Many Vegetables.
Common scab is a disease caused by soil-dwelling, bacteria-like
organisms. Roughened, sunken or upraised discoloured patches occur
on potato tubers. Deep pits on tubers are created when insects attack
early infections. Neutral to alkaline soil pH, fresh manure and
infected potato seed promote this disease.
Check seed for symptoms of scab prior to planting - destroy any
diseased seed. Keep soil pH at or below 5.2. Use well-rotted manure
and certified seed potatoes. Avoid growing susceptible crops for
at least three years in soil where scab has been a problem. Plant
scab-resistant potato varieties such as Ontario, Superior, Onaway,
Russet Burbank (Netted Gem), Norchip, and Sebago. Maintain high
soil moisture for about four weeks after tuber setting begins.
Early blight is a fungal disease occurs annually in most of Ontario
by midsummer. Large, brown target spots with concentric rings appear
on older leaves and spread to younger ones. Lesions are frequently
surrounded by a narrow yellowish border. Potato tubers or tomato
fruit are rarely affected, but if the calyx of the tomato is infested,
fruit will drop before ripening. If infected, tubers develop dark
brown to black, slightly sunken external lesions. The fungus overwinters
on diseased residue.
Follow proper garden sanitation procedures. Avoid overhead watering
and remove and destroy crop debris after harvest. Vigorous plants
are less susceptible.
To control leafhoppers, Colorado potato beetles and blister beetles
as well as early blight and late blight diseases without pesticides,
plant early or medium-early varieties of potatoes. Presprout seed
potatoes in trays under light during April and plant in early May.
An acceptable crop can be harvested by mid-July, before extensive
development of the blight.
Hollow heart is an abiotic disorder that can affect large potato
tubers. When cut in half, the centre of the tuber shows an irregular
hollow area. This cavity is usually outlined in brown, and if the
cracks originating at the cavity extend to the surface, the tuber
may rot. As a rule, there is no decay involved and the tubers may
be used with a little extra trimming.
Any condition such as over fertilizing that induces over-sized
tubers can cause hollow heart. Closer spacing,
less nitrogen and less water will aid in avoiding this disorder.
Avoid over-watering when potatoes are sizing. Severity will vary
from year to year depending on growing conditions. This condition
is favoured by cool, wet soil conditions at flowering.
Late blight is a fungal disease that occurs under wet, cool weather
conditions during late August and September. It is usually worse
on low-lying, wet land and heavier clay or clay loam soil. Initially,
small light to dark green water soaked spots develop on the leaves,
and eventually become grey to tan, surrounded by a light green halo.
Under humid conditions (e.g. early morning), a white, fuzzy fungal
growth often develops on the underside of the leaf. Lesions on tubers
are brown to purple and slightly sunken, with a reddish brown rot
developing under the skin. Eventually, the entire tuber can rot
from a combination of the blight and secondary bacterial infections.
The disease originates from infected seed potatoes. Late blight
has the potential to be an extremely destructive disease of potatoes
in Ontario. This disease can also affect tomatoes, and late blight
can be introduced to gardens on infected tomato transplants.
Use only certified seed. Destroy volunteer potatoes and control
host weeds growing nearby. Ensure plants can dry quickly - avoid
overhead watering, water early in the day, do not crowd plants and
plant in a sunny spots. Check plants regularly - at least twice
a week. Destroy tubers showing dark spots when cutting seed potatoes.
Do not compost. For tomatoes, remove and destroy lower infected
leaves. Widely space, prune and stake tomato plants. Once the disease
shows on leaves, it spreads rapidly. Harvest the potatoes immediately,
if infected in late summer, by cutting stems at ground level and
waiting two weeks before digging to allow vines to completely dry
so fungus spores are not sprinkled on potatoes. If contact is made,
potatoes will rot in storage. Vines can be placed on compost. During
digging, discard and destroy tubers with brown spots under skin
- do not store infected tubers.
Late blight can be introduced to gardens on infected tomato transplants
and infected seed potatoes. Inspect all tomato transplants and seed
potatoes for signs of disease before planting. Do not plant any
diseased plant material. Destroy all transplants and seed potatoes
To help control late blight, plant early or medium-early varieties
of potatoes. Presprout seed potatoes in trays under light during
April and plant in early May. An acceptable crop can be harvested
by mid-July, before extensive development of the blight. This will
work only if the seed potatoes are not themselves infected with
The leafhoppers affecting potato are pale green and about 3 mm
long. For more information and additional management options, refer
to Chapter 1
and Chapter 4.
To help control leafhoppers, Colorado potato beetles and blister
beetles as well as early blight and late blight diseases, plant
early or medium-early varieties of potatoes. Presprout seed potatoes
in trays under light during April and plant in early May. An acceptable
crop can be harvested by mid-July, before insects have caused such
extensive foliage damage that no further tuber growth is possible.