Bean Diseases and Their Control

Most bean diseases and their control in Uganda come down to one question asked early: which pathogen is actually in your field. Altitude, temperature and how wet the canopy stays decide that, and a variety sold as tolerant may be tolerant to something else entirely. A survey of all seven Ugandan bean agroecologies found root rots everywhere, with a different one dominant in the highlands than in the lowlands.

The commonest mistake is not failing to spray. It is buying a resistance claim that covers the wrong organism, then treating the result as a spray failure.

Where You Farm Decides Which Bean Disease You Get

Bean pathogens are narrow about conditions. CIAT's bean disease handbook, written by Buruchara, Mukankusi and Ampofo for small scale seed producers, records the conditions that favour each one, and reading them side by side turns a long list into a short decision.

Bean Root Rots: Southern Blight Has Taken Over

Bean root rot is not one disease. It is a complex of soil borne fungi, and which member dominates has changed in Uganda within a generation of farming.

The first systematic Ugandan survey of the four main root rots found all of them present in all seven bean agroecologies. Ranked by how widespread they were, southern blight caused by Sclerotium rolfsii came first, then Fusarium, then Pythium, then Rhizoctonia. Southern blight reached 100 percent prevalence in Bugiri, Busia, Butaleja and Jinja districts, meaning every farm surveyed there had beans wilting from it, and its highest recorded incidence in a district was 84 percent of plants in Bugiri.

Earlier work had put Fusarium and Pythium in the lead, especially in high altitude areas. Severe root rot in the low and medium altitude zones, where conditions are dry and warm, is the newer picture, and southern blight is what changed.

A second Ugandan point worth holding on to: root rots rarely arrive alone. Just over half the surveyed fields, 52 percent, carried a single root rot pathogen. Another 34 percent carried two, 10 percent carried three and 1 percent carried all four, and individual plants were infected by more than one. Two of them make each other worse, with published work finding that Fusarium and Pythium together do more damage than either alone.

The losses are large enough to matter. Root rots are estimated to cost sub Saharan Africa about 221,000 tonnes of beans a year. Field yield losses from southern blight alone have been reported at 55 to 65 percent, and from Fusarium root rot at up to 86 percent in severely affected soils.

What a Root Rot Tolerant Bean Variety Does Not Cover

This is the part that changes what you buy, and it is the strongest Uganda specific finding in the bean literature.

None of that makes variety choice pointless. Released Ugandan varieties carry documented tolerance to anthracnose, angular leaf spot and bean common mosaic virus, and those are real diseases with real losses. CIAT's handbook names two Ugandan lines, recorded as NABE 13 and NABE 14 with the synonyms RWR 1946 and RWR 2075, among varieties for root rot resistance. What has to go is the belief that a tolerance claim covers the whole root rot complex. Ask which pathogen, and if the answer is southern blight, treat the claim with caution.

The Bean Wilting That Is Not a Disease

Before spending anything on a root rot, rule out an insect, because the Ugandan survey team ran into this themselves. Confirming their field observations by isolating pathogens, they noted that wilting in several instances was caused by the bean stem maggot rather than by any root rot at all.

Bean stem maggot, also called bean fly, is three species of Ophiomyia. The adults are small shiny black flies about 2 mm long whose clear wings flash metallic blue in sunlight. The first sign is egg laying punctures on the primary leaves, concentrated near the leaf base. Maggots hatch and tunnel down the stem to the root zone, where they pupate as brown or black cases beneath the stem or root skin. That cuts nutrient movement, the tap root dies, and the plant either wilts within days or survives stunted, trying to grow adventitious roots above the damage.

The diagnostic difference is what you find when you dig. A root rot shows lesions, discoloured or pruned roots, rotted tissue or white mould and sclerotia at the stem base. Bean fly shows a tunnelled stem and small pale maggots or hard dark pupal cases under the skin. Both plants wilt identically from above.

Bean fly damage is worst where the crop is already stressed: infertile soils, drought or moisture stress, and soils carrying disease. Populations peak late in the season, so early sowing avoids the worst of them, and repeatedly planting beans on the same plot builds the population up. Earthing soil up around the plant at first weeding helps damaged plants form new roots, and mulching with material such as rice straw has the same effect.

Angular Leaf Spot, and a Resistance Claim That Fails in Uganda

Angular leaf spot is caused by Phaeoisariopsis griseola and it is one of the diseases Ugandan varieties are most often described as tolerant to. It shows as brown spots with a tan or silvery centre, boxed in between the major leaf veins, which is what gives the lesions their angular outline. Symptoms are most obvious at late flowering and early pod formation. Some varieties carry a yellow halo round the lesion, the whole leaf then yellows and drops early. Pod lesions are sunken, irregular to circular, black with reddish brown centres, and can be confused with anthracnose.

Here is the finding worth carrying to a seed counter. CIAT's handbook lists three lines as resistant in Malawi, South Africa, Tanzania and Zambia, and then states that those same lines are susceptible in Uganda, naming a different set as the resistant ones for Ugandan conditions. Angular leaf spot resistance is not a property of the variety alone; it is a property of the variety against the pathogen population where it is planted.

That is the general shape of the problem with resistance claims in East Africa, and it is why a variety catalogue from a neighbouring country is not a shopping list for Uganda. What travels better is the non chemical set: rotate with a non host such as maize for at least two years, remove or deep plough crop debris and volunteer bean plants, stay out of the field while the leaves are wet, and plant mixtures that include a resistant line to slow the spread.

Bean Anthracnose and Why Seed Is the Main Carrier

Anthracnose, caused by Colletotrichum lindemuthianum, is among the most damaging bean diseases worldwide and it likes frequent rainfall with cool to moderate temperatures and high humidity, which describes a great deal of Uganda.

Look for dark brown to black lesions running along the veins on the underside of leaves, and on petioles and stems. The pod symptoms are the clearest: sunken lesions ringed by a slightly raised black ring with a reddish border outside it. Under heavy infection young pods shrivel and dry early.

The reason anthracnose deserves more attention than its symptoms suggest is what happens next. The fungus can penetrate the pod, discolour and distort the seed, then establish itself inside the seed coat. Planted, that seed becomes the source of infection for the following crop. CIAT's handbook says the best control may be simply using disease free seed, which is a strong statement for a document that lists chemical options.

Alongside clean seed: rotate with non hosts such as cereals for at least two years, remove or bury plant debris soon after harvest, keep cultivation and spraying out of a wet crop, avoid overhead irrigation, and clean seed storage before putting a new crop in. Staking climbing beans also helps, because climbing the stake lifts foliage into the air and the Bean Cultivation Handbook names reduced anthracnose attack as one of the benefits.

Telling the Two Bean Bacterial Blights Apart

Uganda has two bacterial blights on beans and they split by temperature, which makes them easier to separate than their symptoms suggest.

Common bacterial blight, caused by Xanthomonas phaseoli, likes warm to high temperatures with high humidity. It starts as water soaked spots on the underside of leaves that enlarge and merge into large brown irregular lesions edged by a narrow yellow zone. The yellowing then spreads across the leaf. Pods carry sunken circular spots, water soaked at first, drying to leave a narrow reddish brown border, and under wet conditions yellow slimy bacterial ooze forms a crust on the lesions.

Halo blight, caused by Pseudomonas savastanoi pv. phaseolicola, is the cool weather one: favoured by temperatures of 16 to 20 C with moist cloudy conditions, multiplying quickly where there is dew, and uncommon in hot regions or hot seasons. It is more prevalent in mid to high altitude areas. It starts as tiny water soaked pinprick spots on leaf undersides whose surrounding tissue turns yellow green, forming the halo that names it. Branches and petioles carry greasy spots that redden.

Both stem symptoms converge in a way worth knowing: either can rot the stem at the first node, where the seed leaves were attached, and snap the plant there.

Both are carried on seed and in debris, and both spread by water splash, windblown rain and by people and tools moving through a wet crop. For common bacterial blight, CIAT's handbook carries a sentence that should change behaviour: even a trace of infected seed, when planted, can start severe infection of whole fields. Rotation of about three years away from infected plots, deep ploughing of debris, clean seed, and suspending field operations while leaves are wet are the measures that do not depend on a product.

Bean Rust and the Finger Test

Rust, caused by Uromyces appendiculatus, occurs wherever beans are grown, and it is the one bean disease with a field test anybody can do.

Rub the spots. Rust spores come off on your fingers and blights do not. The pustules enlarge into reddish brown or rust coloured raised spots that leave a rusty mark on anything they touch, and under heavy infection leaves brown, curl upward, dry and drop early. Green pods and sometimes stems and branches carry the same pustules, and early severe infection cuts pod set, pod fill and seed size.

Rust needs cool to moderate temperatures and wet conditions that leave free water sitting on the leaf for more than ten hours, and plants infected early lose the most. There is a second condition worth noting, because it links this page to fertilizer: anything that delays maturity raises the potential loss when a rust epidemic hits, and the handbook names late planting, herbicide damage, excess nitrogen and hail damage in that list. So overfertilising beans with nitrogen carries a rust risk on top of the nodulation problem covered in our bean fertilizer guide.

Rust spreads mainly on the wind rather than on seed, so its management is different from anthracnose. Rotate two to three years with cereals, deep plough infected debris, remove volunteer bean plants and other spore sources, plant within the recommended window to avoid late season spore loads, and watch the crop closely through flowering and early pod development.

Bean Common Mosaic Virus and the Aphid Link

Bean common mosaic virus and bean common mosaic necrosis virus are the two most damaging viral diseases of beans in Africa. Both show a mosaic of light green or yellow against dark green on the leaves, with puckering, distortion and rolling, and often mottling, malformation and general stunting. Plants infected early, or grown from infected seed, mature late and carry fewer pods with fewer seeds in each.

The necrosis virus adds a distinct and more alarming symptom in susceptible varieties: black root disease, which appears as diffuse reddish brown areas on young trifoliate leaves with reddish brown to black veins, darkening of stems, pods, seeds and roots, then wilting and dieback at the growing point.

Control has two levers and neither is a spray on the virus. The first is resistant varieties, which CIAT's handbook calls the best control measure for the mosaic virus, along with keeping susceptible varieties out of areas known to carry the necrosis virus. The second is breaking the transmission chain. The major source of infection is infected seed, and spread in the field is by several species of migrating aphids, so planting clean seed, planting early to miss the aphid peak, and destroying other legumes and weed hosts before and during the crop are the practical measures.

Aphids matter here more as couriers than as feeders. CIAT's handbook is explicit that the direct damage from aphid colonies may not be as economically important as their ability to transmit the mosaic virus, and that several aphid species which never colonise beans are suspected of carrying it while passing through. Aphid infestations get worse in dry conditions, and in humid weather large colonies can be wiped out by insect attacking fungi without any help.

There is a trap in controlling them. Lacewings, lady beetles, the minute pirate bug, hoverflies and parasitic wasps keep bean aphids in check in the field, and the handbook warns that certain insecticides kill the aphids and the natural enemies together, making the problem worse. The same warning is recorded for whitefly, which is not well controlled by any available insecticide and becomes a problem mainly where persistent spraying has already removed its predators.

The Bean Leaf Diseases That Spacing Controls

Four bean diseases have wide spacing written into their management, which means the disease decision was made on planting day.

Web blight is the aerial form of the same fungus that causes Rhizoctonia root rot. It is sporadic and minor across most of Africa but serious in humid lowland areas with high to moderate temperatures. The symptom is unmistakable: leaves look as though they were scalded with hot water, grey green to dark brown, and small water soaked spots expand until the plant appears burnt, with brown sclerotia and light brown fungal threads webbing over it. Its host range runs past 200 species and includes tomato, cucumber, cabbage and watermelon, which has consequences for rotation.

White mould, caused by Sclerotinia sclerotiorum, is widespread but only seasonally important under cool moist conditions at middle altitudes. It attacks old or injured tissue, flowers, cotyledons, pods, seeds and leaves, producing greyish green water soaked lesions that turn to white cottony mould with a watery soft rot, then black resting bodies in and on the tissue. Mulching to keep pods off the soil is one of its listed controls.

Ascochyta leaf spot shows as large dark grey to black spots that become zonate, with concentric rings holding black fruiting bodies. Its stem symptom is more dangerous than its leaf symptom: blackening at the nodes can girdle the stem and kill the plant outright.

Powdery mildew and floury leaf spot are easily confused and separated by one observation. Powdery mildew's white mould sits on the upper leaf surface; floury leaf spot's sits on the lower surface, with yellow or green and brown spots showing through on top. Powdery mildew likes warm temperatures of 20 to 24 C with low humidity and shade and is minor in Africa. Floury leaf spot is widespread in eastern Africa at mid to high altitude under cool to moderate temperatures with high humidity, which makes it the more likely of the two in a Ugandan highland field.

For web blight, white mould, Ascochyta and powdery mildew alike, the handbook prescribes wide spacing to increase air movement and mulching to stop rain splash carrying spores between plants. Those are decisions taken at planting, and they sit alongside the plant population arithmetic on our page about bean spacing and seed rate per acre.

Rotation Only Works Against a Verified Non Host

Almost every disease on this page lists crop rotation among its controls, usually two to three years with a non host, and cereals such as maize or sorghum are the usual suggestion. That advice is sound and it has a hole in it that Ugandan research has now identified.

Southern blight has a wide host range, and work testing strains against other crops found them able to attack tomato, eggplant and chickpea while leaving chilli, soybean and cowpea alone. The conclusion drawn from that is precise: because not all strains attack all of the pathogen's hosts, farmers with limited resources have to pick rotation crops that are genuinely non hosts for their own situation if rotation is going to lower the inoculum in the soil.

Which means a bean and tomato rotation, common enough on Ugandan smallholdings, may not be a rotation at all as far as southern blight is concerned, and web blight's 200 species host list includes tomato, cabbage, cucumber and watermelon too. Cereals are the safer partner because they appear as non hosts in the management notes for southern blight, angular leaf spot, anthracnose, rust, powdery mildew, charcoal rot and Fusarium wilt alike.

The other soil measures carry across most of the root rots: deep ploughing of infected material during land preparation, letting the previous crop decompose fully before planting, planting on raised beds or ridges for drainage and aeration, hilling soil up around the stem before flowering to encourage new roots, and building soil fertility with farmyard manure, green manure or compost rather than with heavy nitrogen.

Deciding When a Bean Crop Is Worth Spraying

This page names no product and no rate, for a reason worth stating. The Ministry of Agriculture's own list of registered agrochemicals was unreachable while this page was written, two NARO bean documents give the same seed dressing at rates differing by a factor of ten, and a bean seed dressing named in both is a compound withdrawn in many countries. Under those conditions the responsible thing is to publish the decision rules and leave the product to the label and to a qualified adviser.

What is publishable, and genuinely useful, is when to act. The Bean Cultivation Handbook carries scouting thresholds rather than a calendar spray programme.

Where a chemical measure is genuinely the answer, the classes involved are copper based materials for the bacterial blights and protectant or systemic fungicides for the leaf spots and rusts. Every rate, every dilution, every interval between sprays and every pre harvest interval belongs to the label of a product registered for beans in Uganda, and both the choice and the timing should be confirmed with an agricultural extension officer before you buy. Our pages on how fungicides work, reading an agrochemical label and applying pesticides safely cover the ground a product page cannot.

Biological options appear in CIAT's handbook alongside the chemical ones and carry fewer risks. Neem extracts deter egg laying and disrupt feeding in many bean pests, formulations of Trichoderma and Pseudomonas species are described as effective against a range of root and stem rots, and farmer prepared concoctions from neem seed kernel, tephrosia leaf, fermented cattle dung and urine, tobacco leaf, chilli and garlic are listed as the commonly used preparations. Treat those as an integrated programme rather than a replacement for clean seed and rotation.

Bean Diseases That Follow the Crop Into Storage

Several bean pathogens finish their work after harvest, and one pest splits neatly by altitude in a way that changes what you do.

Bean bruchids are two species. The common bean weevil is more frequent in high altitude or cool environments and is the one more often found in eastern Africa, and its infestation starts in the field, with eggs laid in pods before harvest. The Mexican bean weevil prevails in warmer environments and its infestation normally starts and multiplies in storage, with white eggs glued to the seeds. Larvae of both bore into the grain, leaving it perforated and unfit for planting or eating.

The split matters because the field starting species can only be avoided in the field. Harvest early, before splits appear in the pods, and you cut the common bean weevil off before it reaches the store. For both species: dry the seed thoroughly before storage, sort out damaged grain, boil used sacks in hot water and dry them in the sun before reuse, keep new harvest separate from old, clean and burn trash out of the store, and inspect stored grain regularly.

Two mechanical measures are worth more attention than they get. Tumbling seed in a sack, or rolling it in a drum, crushes the eggs on the seed surface and stops new larvae getting in, with no chemical involved. Hermetic storage kills insects already in the grain by suffocation, holds beans safely for more than a year and keeps seed viable. Spreading seed in the sun under plastic covers also kills pests by heat, but it is unsuitable for seed you intend to plant, because the same heat damages germination.

Fumigation is licensed work. The target has to be sealed, protective clothing is needed, the grain and store must be aerated afterwards, and only licensed people should do it. Storage methods in full are covered on our bean storage guide.

Getting a Bean Disease Identified Before You Buy Anything

The sequence that saves money on this page is short. Pull up several wilting plants and look at the roots and the stem base, so you separate a root rot from bean fly and from a leaf disease that has simply gone far. Rub a leaf spot to see whether it is rust. Check whether the mould is on the upper or the lower leaf surface. Note whether your site is cool and cloudy or warm and moist, which narrows the list to two or three candidates. Then take a sample to a district agricultural extension officer before buying a product, because at least two of the commonest Ugandan bean root rots have no resistant variety and several of the leaf diseases are managed better by clean seed and rotation than by anything in a bottle.

When you do buy, buy seed first. Anthracnose, both bacterial blights, angular leaf spot, ascochyta, powdery mildew and both mosaic viruses are all carried on or in the seed, which makes seed class the single most useful purchase on this list. Our guide to choosing bean seed in Uganda covers seed classes, and you can compare what pesticide and fungicide suppliers stock near you under pesticide suppliers. The rest of the bean season sits in our crop growing guides.

Questions Farmers Ask About Bean Diseases and Their Control

Bean disease control in Uganda rewards diagnosis far more than it rewards spending. Establish whether the problem is above or below ground, whether it is a fungus, a bacterium, a virus or an insect, and whether your site is the cool cloudy kind or the warm moist kind. Then buy clean seed of a variety whose tolerance claim names the pathogen you actually have, and take a sample to a district extension officer before choosing any product or rate.