DISEASES OF FRENCH OR DWARF BEANS
By G. B. Wallace, B.Sc. (Agric.), Ph.D., Plant Pathologist
This article is written particularly for the large-scale European
grower, on whose extensive fields disease can sometimes attain
significant proportions. Diseases in beans of the African grower
are not spectacular, as the plots are generally small and scattered,
and the beans are often interplanted with other crops. The African
bean grower will best be assisted by watchfulness on the part of
Agricultural Officers, who should look out particularly for. the
seed carried diseases, and when these are found recommend a source
of clean seed from neighboring healthy plots.
French or Dwarf beans (Phaseolus vulgaris) are an important
crop in Tanganyika both for food and for the production
of seed beans for export. Diseases of the crop are significant,
first because, they cause loss in crop, and secondly because diseased
seed may be exported. No complaints have been received on the
latter score, but the risk is patent and it is the aim, of all concerned
that the danger of exporting any seed-carried disease be reduced
to the absolute minimum. It is thus essential that the. growers be
able to recognize the bean diseases and know what measures are
practicable for their control. Both the Tanganyika Export Seed
Growers' Association and the Department of Agriculture have in
mind the need for a regular inspection service on the seed farms.
The causes of bean diseases are various: they are mostly fungi,
but one virus and one bacterium are present, while a few are
hereditary weaknesses in the plant. Diseases resulting from
deficiency of particular chemicals in the soil are probably more
common than has been realized, and opportunities will be taken
to investigate such. The significance of individual diseases is
influenced to a large extent by local weather or atmospheric
conditions, and this can be partly off-set by the time of sowing.
The diseases of French beans are mostly confined to that crop,
and with the exceptions to be mentioned, spread from or to the other
types of beans is not serious. The diseases which have been
recorded in the Territory are described below. Their relative
importance varies considerably from year to year and often from
farm to farm; each one has been abundant and destructive at one
time or another.
In Table I the diseases,of beans are listed according to the parts
of the plant affected. Table II shows the sources of infection and
the methods of spread : it will be noted that three diseases are
transmitted by seed, and that two of these can also originate in
infected soil. Table III is a summary of control measures.
Only passing mention need be made of certain fungi which are
occasionally found causing leaf spots without doing much damage;
an example is Mycosphaerella ?pinodes.
TABLE I.-Bean Diseases: Parts of plant showing symptoms.
Halo blight ..
Yeast spot ..
Burst skins ...
Mosaic ... .
Halo blight ...
Angular spot ...
Sclerotinia rot ...
Yeast spot ...
Burst skins ...
+ + +
+ + +
+- + +
in diseases : Sources
Source of infection
seed (as Soil Wind seed i
+ + +
of infection and methods of
Spread of disease by
d T Encou-
d Inheri- Wind InsectsContact raged
ted by rain
+ + +
TABLE III.-Bean Diseases: Methods of control.
Halo blight .. ... (+)
Anthracnose ... ... +
Angular spot ... ... +
Yeast spot ...
Rust .. ... ... +
Burst skins ... .
and avoid damp
Do not sow too
early in rains
for dry beans
This is the only bacterial disease of French beans so far
recognized here. Others are known elsewhere but the control
measures are much the same for all. Halo blight is not common
in the Territory, but it has occurred in various parts, sometimes
severely. In the 1948 season it was observed on two farms in
serious proportions, and in moderate amount on others.
The disease is a matter of particular concern to all bean growers,
first because it spreads rapidly from small initial infections, and
secondly because it is seed carried and infected seed cannot all be
recognized in the process of grading. It has so far been seen only
in French beans, but Runner and Lima beans are. also susceptible.
Infection of a crop starts in any one of three ways : (1) by the
use of infected seed; (2) from soil which has borne a diseased crop
within the previous year; (3) by bacteria brought in by wind,
contaminated dust, implements or possibly on the clothing of
Symptoms.-On the leaves the earliest and most characteristic
symptom is small brown spots each surrounded by a pale green
"halo" one-half to an inch across (Fig. 2). Several brown spots
may be surrounded by one. large pale area. In systeniic infections
brown areas often with a yellow border may cover a large part of
a leaf. Such leaves tend to be cupped downwards, and soon wilt
and fall (Figs. 1 and 3). Another symptom is wide pale green
margins to the leaf with normal green within. The youngest top
leaves of an affected plant are pale and can be recognized some
distance away; the paleness results from the lighter green to almost
colourless tissue between the veins in these leaves.
The necrotic areas associated with this disease can be distin-
guished from those of Angular leaf spot by the absence of the tiny
erect black spore-bearing bodies found in the latter. The cupping
of the leaflets may be confused with that seen in Mosaic and "Sun
scorch", but the other symptoms will distinguish these, diseases.
On the stems dark green to red streaks indicate bacterial
infection; they can be distinguished from the grey to brown streaks
of the Angular leaf spot fungus by the presence of a bacterial ooze
and by the absence of fruiting bodies.
The pod infection is characteristic (Fig. 5): round dark green
"water soaked" areas appear and become slightly sunken; a rusty
red colour may develop round their margins or over the entire
surface of the spots. At their centres a light cream-coloured slime
of bacteria may be seen oozing out. The sutures of the pods may
show the dark green and red colour throughout their length, and
the bacterial ooze may be seen there also. The spots are obvious
even in dried pods.
Seeds which are seriously infected (Fig. 4) can be recognized by
their flatness and the wrinkling of the skin (wrinkling can also be
caused by moisture on the seed). In unripe green seed, water-
soaked areas can sometimes be found; their surface is often covered
with a sticky layer of bacteria which later dries as pale scales.
Less severely affected seed cannot always be recognized, and the
only satisfactory criterion for healthy seed is the health of the
Control.-For stock seed to be sown on the seed farms the seed
should be taken only from fields in which no Halo blight has been
observed after thorough inspection. Since the bacteria can live in
the soil for at least a year, rotation is most essential where the
disease has been observed. Rotation, apart from its purely
cultural advantages, should be a normal practice as a precaution
against soil infections generally. Field operations should be done
when the plants are dry to avoid unnecessary transfer of bacteria,
and also of the spores of other diseases to healthy plants.
When the disease is found in a field of beans being grown for
seed, a decision has to be made as to what action to take, and that
will depend largely on the extent of the infection and on the age
of the plants. In a young crop, if the infection is extensive, it
would be best to destroy all the plants, while if it is restricted
the affected plants should be removed and burned, and the crop
inspected thereafter at regular intervals for further cases.
Should the disease become extensive at a later stage, roguing
would necessarily have to cease, and the crop be harvested for
food only. Exporting or sowing seed from an infected field entails
risk and should be condemned.
Threshing and cleaning machinery should be. used for healthy
crops first, and diseased crops afterwards.
The considerable amount of Halo blight on two farms in the
1948 season caused some alarm, and pointed to the necessity for
the routine +measures: THE USE OF HEALTHY SEED,
CROP ROTATION and FIELD INSPECTION. By healthy seed
is meant seed from a crop free from all seed-carried diseases : Halo
blight, Mosaic and Anthracnose. There is no doubt that if these three
precautions become universal practice it will be possible within a
few years to guarantee that the bean seeds exported from the
Territory are 100 per cent free from seed-carried diseases.
The question of using varieties of French beans resistant to
Halo blight has not arisen as seed growers are provided by the
seedsmen abroad with the varieties to be multiplied. Should the
necessity arise for resistant varieties, several have been listed
elsewhere and might be tried; it has however been observed that
one of these, the popular Prince, has not proved resistant in this
Seed for multiplication on the farm should, ideally, be raised on
a separate isolated plot. Such a plot should be given special care,
including regular inspection; wide spacing of the plants would
permit readier observation of diseased plants and of rogues and also of
particularly good strains. The expense of spraying would be
justified on such a plot. Wide spacing would also discourage such
a disease as Sclerotinia rot, as well as the plant to plant infection
by any Halo blight.
Plants from seed newly imported for multiplication should receive
particular care in inspection for seed carried diseases, for there are
several virus and bacterial diseases of beans not yet observed in the
Territory, which may yet appear.
As regards seed disinfection for Halo blight, investigations
elsewhere have shown that this is not satisfactory, but where there
is any doubt about the cleanness of seed, a dressing with a
mercurial dust would be advisable. Such dusts are obtainable
locally; they are used at the rate of about 3- oz. per cwt. and cost
under Shs. 2/- per pound; the makers' instructions should be
Spraying bean plants with Bordeaux mixture is of doubtful value
economically, but should it be decided to spray for Rust or Angular
leaf spot, the treatment would assist also in protecting the plant from
outside infection by Halo blight.
This is a virus disease which is transmitted in the seeds and
which is spread in the field by aphids. It has not been seen on
the epidemic scale seen sometimes with Halo blight. It is present
however in all districts and although usually insignificant to
moderate in extent, it is potentially serious and should be actively
controlled. A few of the symptoms of Mosaic may be confused
with some found in Halo blight, such as cupping of the leaflets,
but when a number of symptoms are compared there need be no
Plants affected early by the virus remain small and stunted with
bunched foliage (Fig. 9). The leaflets remain small but the
petioles may grow to normal length. The leaflets are distinctly
mottled with yellow, pale green and dark green; they are usually
malformed, blistered, puckered and cupped downwards and may be
narrow. Pods are produced in small numbers and are likely to be
small, twisted and warted. There are no recognizable symptoms
in the stems or seeds. Symptoms have been said to be more
pronounced at high than at low temperatures.
Among the known aphid vectors are Aphis rumicis (the black
bean aphid) and Myzus persicae (the peach aphid), which occur
in East Africa. The virus is seed-borne and it is recorded that
about a third of the seeds from infected plants transmit the
disease. Pollen can also transmit the virus. It appears likely
that this virus is restricted to French beans.
Control would be best ensured by the use of resistant varieties,
of which several have been produced in the U.S.A. Control by
roguing cannot be fully effective, but under our conditions of light
to moderate infection, it is advisable and should commence early
in the season and be undertaken throughout a district if possible.
Seed for multiplication should be taken from fields known to be
In 1941 a form of Yellow bean mosaic was seen in a number of
imported varieties, but these were not sown again and the disease
has not reappeared. It is not seed-borne. The symptoms are much
like those of Common mosaic but the plants are more yellowed and
the leaflets are cupped upwards. It is carried by aphids and spread by
contact. A number of other leguminous host plants are known.
ANGULAR LEAF SPOT
This is the commonest disease of beans in the Territory. While
normally of little importance, it can be severe in some years as in
1947 when the rains were prolonged and the disease started early.
Both quantity and quality of the crop are affected. The fungus
is not transmitted in the seed.
All the above-ground parts of the. plant are affected and the
symptoms are very distinct in each. They are most obvious first
in the leaves and later in the pods. The leaves show a varying
number of angular spots on both surfaces (Fig. 6); they are
delimited by the veins and are of a grey to brown colour. On the
lower surfaces of these spots there can be seen a dark dust which
when magnified is seen to be a layer of erect brush-like bodies.
Each of these consists of a narrow stalk bearing a loose, mass of
spores mostly at the top. The stalks arise from tiny black bodies
known as sclerotia. On older leaves only the sclerotia may
remain. Affected leaves dry up and fall early with more or less
severe effect on the cropping. Even in a good season this disease
in the leaves may be found in some abundance when the plants
are drying up, but by that time the pods and seeds are formed and
there is little damage.
On the pods the fungus causes rounded red-brown areas with
indefinite grey-green margins; they vary in size. but are about a
half to two-thirds of an inch in diameter (Fig. 8). On the surface
of all but the older spots the fungus fructifications can be seen,
similar to those on the leaf spots.
The. fungus can penetrate a pod and reach the seeds; such
infected seeds are discoloured, pale or brown, but most of the damage
is indirect through leaf fall and pod infection. Raised orange
blisters which have been attributed to this disease, have been found
recently to be associated with the Yeast spot fungus, Nematospora,
which is described below. The stem and branch lesions are brown
and according to the freshness of the material they also show the
Control.-The origin of outbreaks of this and some other diseases
each season is almost certainly from infected plants growing out of
season. There is therefore a good case for restricting the planting
of beans to one time of year, sowing only with the end of the rains.
Where such is done an effort should be made also to destroy bean
plants coming up in the fields from self-sown seeds.
If it is anticipated that wet conditions are likely to be prolonged,
the sowing date might be postponed, though not unduly. Spraying
with Bordeaux mixture or dusting with sulphur would be beneficial
and on multiplication plots at least the expense involved would be
justified. Spraying or dusting should be commenced early and be
repeated a few times until the flowers start opening, after which it
should be discontinued. The observed spread of the disease from
early to later sowings indicates that sowing dates should not be
PLATE I-HALO BLIGHT
I.-Plant with Halo blight
3.--Leaf showing cupping
2.-Leaflet with brown spots and
4.-Seeds of two varieties flattened
and wrinkled by Halo blight; two
seeds are healthy
5.-Pod with lesions
6.-Leaflet with Isariopsis spots
7.-Seeds with blisters
of the Yeast spot fungus
8.-Pods with Isariopsis spots
9.--Plant with Mosaic
10.-Leaf with Rust pustules
II.-Pod with Rust pustules
12.-Leaflet with Sunscald
15.-Seeds with "Burst skins"
16.-Seeds with Yeast spot
spread over a long period on any one farm or even on a group of
There can sometimes be seen a white. fungus overgrowing the
angular spots. This is White mould (Hyalodendron or Cladosporium
album), and it is not of much importance; it need not be confused
with the white fungus Sclerotinia sclerotiorum which is described
A fungus disease which may be confused with Angular leaf spot
is that caused by Rhizoctonia bataticola. This has however been
seldom observed. On the pods and stems it causes lesions very
similar to those of Angular leaf spot, but it has no raised brush-like
fructifications. The colour of the lesions is slate to black, and they
may be extensive and even up to several inches long. (R. solani is
a different fungus which can cause damping-off of seedlings and
root rot in older plants).
Up to the present this disease, although fairly widespread, has
been only moderately serious in Tanganyika. There was, however
one serious outbreak on a farm in early 1950 which showed the
potential destructiveness of the disease.
The infection of a bean crop arises from the use of infected seed
or from soil which has borne a diseased crop in the previous few
years, or from wind-blown spores. The disease is encouraged by
humidity and low temperatures; for that reason it is the practice
in the U.S.A. to grow crops for stock seed in the arid states.
Disease of the pods and seeds is the most characteristic and
damaging, but it must be noted that all parts of the plant are
susceptible and that the pod infection arises mostly from spores
borne on spotted stems and leaves. It is important therefore to be
able to recognize the disease on these parts, and to take what pre-
cautions are possible to control the disease in the early growth of
Seed, which are infected through pod lesions, show yellow or
brown sunken cankers; these may be small or large and they may
bear spore pustules; such seed may also be malformed. The seed-
ling may show the disease in the stem-the hypocotyl-when it
becomes infected from the cotyledons. The disease on the
hypocotyls is seen as rust-coloured spots which become elongated
and sunken and bear spores which can infect the leaves. Care is
necessary to distinguish insect injuries : reddish sunken scars caused
by biting insects have been seen on the cotyledons and hypocotyls
of seedlings. In such a case microscopic examination reveals no
On the stems there are to be seen dark spots increasing in size
to brown patches; these become sunken and bear numerous pale
spore pustules. When the leaf stalk is infected the leaf may droop
and not recover. On the underside of a diseased leaf the veins show
broken lines of a dark red colour, later turning black, and they may
show lesions. Leaves may also show pale translucent spots of
varying size and the infected tissue is liable to dry and fall out;
they bear numerous spores.
The pod lesions are small, one-tenth to a quarter inch in diameter;
they are round and sunken (Fig. 13). The colour is brown with a
definite black margin often slightly raised; the centre may become
pale or even white. It tends to dry out and leave an open wound.
With a lens the spores of the Anthraenose funigus can be seen as
tiny raised pink jelly-like masses which later turn brown or black.
It should be noted that the. spore pustules of the Rust fungus on
pods are rather similar but have a cacao-coloured dust of spores;
in any case of doubt a microscopic examination should be made.
Control.-Preventive measures are definitely more satisfactory
than remedial measures: they are simpler, much less costly and
more reliable. In the first place the seed should be healthy, i.e. they
should be derived from a healthy crop; secondly the seed should be
sown on land which is uncontaminated by the fungus. After a
diseased bean crop other plants should be sown for at least two
years, since it is known that the fungus can persist in the soil for
Unless the grower knows that his seed are from a healthy crop,
it would be advisable to treat the seed with a fungicide such as
Ceresan,-the latter at 2.5 ounces per hundredweight. Such
treatment has been stated to increase the percentage and earliness
germination as well as giving healthy plants.
Opinions vary on the value of dusting or spraying, but good
results were recently recorded in New Zealand (Review of Applied
Mycology XXVIII, 10, p. 502). The investigators applied
Bordeaux mixture. 3-4-50 and cuprox 5-100 two, three and four
times. The sprays were equally satisfactory, reducing the incidence
of pod infection from 44-76 to between 2-08 and 6-65 per cent, the
differences between the number of treatments not being significant.
The average green pod yields from the sprayed and untreated plots
were 44-7 and 29-4 ounces respectively.
The importance of this well known disease varies greatly in
different years. Many strains of the fungus are known elsewhere,
and no doubt there are several in East Africa also. Varieties
of beans differ in their susceptibility.
Leaf infection is the most common but pods may also show rust
pustules. Symptoms show first on the under-surfaces of the
leaves. The pustules, which are at first tiny, pale and raised,
soon burst exposing rust-coloured spores, and later dark spores
(Fig. 10). The pustules are very small but can be numerous, and
each is surrounded by a pale green ring.
The pustules on the pods contain a mass of spores surrounded
by a ragged frill of epidermis; they are larger than those on the
leaves and may attain a diameter of a quarter-inch (Fig. 11). They
must be distinguished from the lesions of Anthracnose already
Various other kinds of beans are also susceptible, e.g. Lima,
Runner and Tepary beans.
Rust is chiefly disseminated by wind, but can be spread to some
extent by rain splashing the spores. It is not seed-borne.
Spraying with Bordeaux mixture, 4-4-50, or dusting with sulphur
are recommended; either should be started early and should not be
continued after the flowers have opened as sulphur and copper are
said to prevent the setting of pods. Too early sowing should be
This disease is not very common, but under certain circumstances
it can be destructive. Its intensity is more closely related to
environment than to the actual parasitism of the fungus itself. It is
dependent on high humidity and more or less stagnant air; these
are encouraged by prolonged wet weather, close planting, the
presence of weeds and of dense wind-breaks.
The fungus has a wide host range and hollow-stemmed plants
are particularly susceptible. Besides beans, the following food and
ornamental plants have been seen infected locally: sunflower,
potato, tomato, pumpkin, lettuce, pyrethrum, borage, Eschscholtzia,
Dahlia, Tropaeolum and Petunia. Cereals are immune.
The disease is at once recognized by the profuse white mycelium
of the fungus growing on or in the affected plants, by the presence
of large black sclerotia, and by the sodden appearance of the plants.
Affected parts- stems, leaves and pods in the case of beans-
become soft and rotted; leaves become grey, thin and papery; pods
become flaccid and grey (Fig. 14). In the hollow stems the white
mycelium and the black sclerotia can be found.
The sclerotia vary in size from about one-eighth to a quarter-inch
in diameter; some are longer than wide. They are black outside
and pink or white inside, and are made up of a dense mass of
fungus hyphae; they are very resistant and can survive for several
years in the soil. Some will however start to produce their fruiting
bodies after a few months under moist conditions. The fruiting
body is a stalked cup, borne at the. surface of the soil, land in the cup
minute spores are formed. Wind-borne spores alighting and
germinating on a susceptible plant will reproduce the disease.
Control.-The best control is to prevent, so far as possible, the
conditions favouring the growth of the fungus. Areas surrounded
by dense vegetation and also low-lying areas should be avoided.
Dense wind-breaks may have to be thinned, and weeds must be
kept down; the densely-growing varieties of beans require ample
spacing. By the time the disease is observed it is usually too late
to ameliorate the conditions, but it should be attempted. Infected
plants should be collected by hand and taken to the. side of the
field to be dried and burned. This requires care as the sclerotia
are easily detached and fall to the ground; sheets of corrugated
iron can be useful to carry off the diseased plants. After an infected
crop is harvested, deep cultivation will bury many fallen sclerotia.
It is not thought essential to abandon an infected field for susceptible
crops if an improvement in the air conditions can be effected
before another crop is sown; but otherwise rotation with cereals
may be necessary.
The common symptoms in this disease are "yeast spots" and
"blisters". The yeast spots are round concave areas, about one-tenth
inch in diameter (Fig. 16), and they may be dark on pale coloured
seeds. They often show a puncture made by the proboscis of the
green stink bug which carries the infective spores. A section
through a yeast spot shows the part of the cotyledon immediately
below to be separated from the skin, and white with a grey margin.
The second symptom, blistering, is found closely associated with
the yeast spots. Till recently it has been confused with the damage
to seeds caused by the fungus Isariopsis. The affected parts are
however usually raised as brown or orange blisters (Fig. 7), and not
as a rule. simply discolorations. They may be small and scattered or
single and large. Both the yeast spots and blisters may show
tiny black marks. Spores of the fungus are usually to be found in
large numbers in the affected parts of the cotyledon.
There is no spread of the fungus through the plant and infected
seeds can be safely sown. Now that it is realized that the blistering
is also a result of insect-carried spores, the practicability of
controlling the bug should be investigated. DDT as a 1-0 per cent
dust or as a 0-1 per cent spray should be effective if applied several
times, perhaps once weekly, after the bug is seen and until the pods
.start to dry. Agrocide powder could also be tried. Either
insecticide could be added to copper sprays, the Agrocide at the rate
of 4 lb. per 100 gallons.
This is a term applied to seeds where the growth of the seed
coat has not kept pace with the growth of the cotyledons, and
where as a result the tips of the latter are exposed (Fig. 15). The
predisposition to this trouble is a hereditary character and varies
with the variety of bean; the variety Prince is much more affected
than ,any other local variety.
The appearance of the seed is spoiled and germination is low.
The trouble is not important where the beans are grown as a
green vegetable, but only when they are grown for dry shelled beans
or for seed.
As in the case of Burst skins, this is a physiological trouble
common in the Prince variety. It appears to be a hereditary weak-
ness in the skin of the seed. The scars are small, up to one-
twelfth inch long and very narrow. Microscopically the scar
differs in several ways from healthy skin. It is thought that a
split occurs at a very early stage in the development of the seed
coat, and becomes filled with yellow callus cells. The damage is
slight and does not reduce the value of the seeds to the seedsmen,
so that affected seed need not be discarded when grading.
All parts of a plant may be affected by Scald, but the effect
on the older leaves is most typical. Strong sunlight on moist leaves
results in yellow areas between the veins, and later these show
islands of dead brown tissue (Fig. 12). The leaves become thin,
papery and brittle. There is sometimes a considerable amount of
leaf-fall. Pods may show brown or reddish areas on the parts
exposed to the sun. Where there is a fair movement of 'air to dry
the plants after rain, the scald is less common.
"Sun scorch" is a term used for various symptoms which are
probably results of deficiencies of necessary chemicals in the soil.
This subject is under investigation. An unusual nutritional
trouble has been seen on a farm on Kilimanjaro : the symptoms
had a superficial resemblance to some. of those of Halo blight.
There was much yellowing between the veins and towards the
margins of the leaves, later turning brown and withering. This
caused the leaves to be rather cupped. The surface was puckered
as if drawn together by a thread. Applications of a number of
chemicals had no effect, but compost applied at the rates of. 5, 10
and 15 tons per acre was very beneficial.
In conclusion the following comments might be kept in mind.
The ideal means of disease control would be the use of varieties
which are immune or resistant, as well as being satisfactory in
other respects. Breeding of beans for disease immunity has
however nowhere progressed far, and other measures cannot yet
be dispensed with.
The importance of clean seed-that is seed from healthy fields-
has been stressed, for epidemics can arise, from very small initial
Some disease symptoms are common to several diseases, and it
is therefore advisable, when endeavouring to identify a disease in
the field, to observe as many as possible. of the symptoms present.
Insect pests of beans have not been dealt with here, except in so
far as they assist in the spread of disease organisms. It should be
kept in mind that disease symptoms in a plant may result from insect
injury. An example is wilting, which can result from cutworm
at the roots, or bean-fly in the lower stem, as well as from fungus
and bacterial parasites.