Farmnote
Fungal diseases of potatoes
By Robert Floyd, Plant Pathologist, Plant Pathology Branch, South Perth Western
Australia
Early blight, Alternaria solani
Early blight is the most common fungal disease of potatoes, causing severe defoliation
and yield loss to susceptible crops. The most susceptible varieties include Delaware and
Russet Burbank, the most important varieties grown for fresh market and processing
respectively.
Leaf infection is first seen as circular brown spots, which rapidly enlarge, becoming
zonate or target-like (Figure 1).
Multiple infections are common and rapidly cover the whole leaf.
Affected leaves usually hang on the plant, where they serve as an efficient source of
wind blown spores. Tuber infections begin on digging damage, producing depressed leathery,
dark grey lesions (Figure 2) and
developing as a dry rot in storage.
Leaves are infected in cool moist weather, below 20oC and when there is free
water on the leaves. Autumn infections are most damaging, which is when processing
potatoes are bulking. These crops need particular attention.
The disease spreads from neighbouring infected crops, paddock residues and dumps of
infected tubers. Tuber dumps and paddock residues should be buried to reduce new
infection.
Tomatoes are also a host. They may be infected from the potato crop and also may infect
new potato plantings.
Rhizoctonia scurf, Rhizoctonia solani
Rhizoctonia affects shoots, stolons and tubers. The disease develops as a brown canker
that can prevent emergence or weaken young shoots (Figure 3).
Stolons are also damaged, leading to loss of tubers.
The fungus is also seen as a dark scurf on the mature tubers (Figure 4).
This is particularly bad in ground-stored crops, where much of the tuber may be covered.
Affected tubers are downgraded.
Yields are rarely reduced by the disease but appearance is affected. Stem and stolon
cankers often result in crops being very irregular in tuber size, with large and small,
rather than medium-sized tubers being dominant.
The disease is introduced on seed tubers, but once established in a field, infection
may be mainly from the soil.
Prompt digging after tuber maturity is the best method of reducing the buildup of
scurf. Do not feed stock with affected tubers on paddocks that are to be planted to
potatoes, since the disease can spread in dung.
Powdery scab, Spongospora subterranea
Powdery scab only affects roots and tubers; it has no top symptoms. It is common during
the winter, being favoured by wet soil.
During warmer weather, infection is less common than in the winter, but infection may
still occur with excessive irrigation and in wet swamps.
The infection usually begins on the roots where galls are formed. These are only of
importance as a source of further disease on the tubers. Tuber infection begins very
early, when tubers are only pea-sized, and continues while conditions are suitable.
Infected tubers first develop swellings, which later burst, exposing a mass of powdery
spores. Affected tubers are unsightly and are downgraded (Figure 5).
The fungus is spread on infected seed. Once introduced, it remains in the soil for up
to 20 years. Control is difficult.
Rotation is ineffective.
Use resistant varieties to control this disease. Katahdin, Exton, Russet Burbank and
Nooksack have useful resistance and should be grown where infection is known to occur.
Common scab, Actinomyces spp.
Common scab is seen in irregularly irrigated crops, either as prominent galls or as
corky depressions (Figure 6).
Unlike powdery scab, this disease is favoured by dry conditions and is most common on
spring and summer crops. It is less common on acid soils.
No chemical control is available, but effective control is possible with accurate
timing of irrigation to prevent the soil drying out during tuber growth. Varieties
resistant to powdery scab are not necessarily resistant to common scab.
Verticillium wilt, Verticillium dahliae
The common symptom of verticillium wilt is known as ,early dying'. This is a top wilt
with yellowing and early senescence (Figure 7).
The lower stems and tubers of infected plants have a light brown internal ring.
The disease is widespread in all potato growing areas and is expected to become a
problem when susceptible varieties such as Kennebec and Russet Burbank are grown.
Tomatoes and nightshade are also susceptible to verticillium wilt. Do not grow tomatoes
in rotation with potatoes and ensure that nightshade is controlled between crops.
The fungus can persist for some years in infected paddocks on these alternative hosts
without showing symptoms. It is difficult to eradicate without fumigation.
The disease is most damaging where meadow nematodes (Pratylenchus spp.)
Fusarium dry rot, Fusarium spp.
Seed breakdown and rot of harvested tubers (Figure 8) may
be caused by Fusarium species. Rapid and careful curing of cut or damaged tubers is the
best control.
Hold tubers after any damage (harvesting or cutting) in a cool (15'C), moist and well
aerated store to promote healing.
White mould, Scierotinia sclerotiorum
White mould is most likely to be a problem in a rotation where vegetables such as
tomato or beans have been infected. Damage is most often seen in over-vigorous crops with
heavy tops and on continuously moist soil.
Stems rot at or near soil level, and pockets of infected plants may lodge in irregular
patches through the crop.
Skin blemishes, Helminthosporium solani and Colletotrichum spp.
Common defects downgrading fresh market tubers are skin blemishes usually referred to
as 'silver scurf'. Two fungi, Helminthosporium and Colletotrichum, cause similar skin
damage, seen as a greyish cast over the surface. When wet, an air layer under the skin
causes a silvery sheen, giving the common name of silver scurf. Infection continues to
develop on tubers in storage, but is worst in ground storage where conditions are
excessively moist.
Neither fungus causes severe yield losses, although Colletotrichum has been found on
roots of plants affected by drought and dying prematurely.
Helminthosporium is controlled by seed treatment
Late blight, Phytophthora infestans
Worldwide, late blight is the most damaging potato disease.
The first symptoms are irregular green spots on leaves, becoming purplish-black with a
yellowish halo. Tubers are also affected, initially with brownish dry rot, which
frequently softens through secondary fungi and bacteria, leading to a complete collapse.
Pink rot, Phytophthora erythroseptica
Pink rot is a tuber disease that is restricted to the southern swamps, although most of
the heavily infected, poorly drained areas have been withdrawn from cultivation. Affected
plants wilt late in the season and all parts may be infected.
Affected tubers have black surface areas. A firm rot develops, from which water may be
expressed. The cut surface of infected tubers produces a salmon-pink colour in 20 to 30
minutes and becomes black in about an hour (Figure 9).
There is no effective control for this disease, but in swamps, improved drainage
removes the predisposing conditions. Control of irrigation and drainage normally prevents
infection in other areas.
Leak, Pythium spp.
Like pink rot, leak is only seen in conditions of poor drainage. Heavy rain near
harvest may cause temporary waterlogging; which allows the fungus to invade tubers. The
condition may develop after harvest where tubers are held in poorly ventilated boxes in
hot sheds. Cut seed may be damaged similarly if it is poorly stored.
In extreme cases, the whole of the tuber degenerates to a water-filled bag that
ruptures under pressure. In earlier stages, the tissues become spongy and wet. A cut
surface will blacken, but does not get the pink typical of pink rot.
Grey mould, Botrytis cinerea
Although it is a relatively minor disease on potatoes, grey mould is common in vigorous
crops. Damage is usually confined to lower leaves within the canopy. Late in the season,
grey mould often occurs with, and is confused with, early blight.
Gangrene, Phoma exigua
This disease produces small dark depressions of the tuber skin, which enlarge to form
'thumb-mark' lesions during storage.
