Managing Fruit Tree Diseases During El Niño Seasons

El Niño seasons can turn disease management into one of the biggest challenges facing fruit farmers in Kenya. When rainfall becomes unusually heavy or persistent, orchards can remain wet for long periods, soils may become saturated, humidity rises and disease-causing organisms can multiply and spread rapidly.

The problem is not simply that there is “too much rain.” The real concern is how prolonged and irregular rainfall changes the environment around the tree. Rain can splash fungal spores from infected leaves and fruit onto healthy tissues, wash pathogens across the orchard, keep flowers and foliage wet for long periods and deprive roots of oxygen when soils remain saturated.

For some diseases, these conditions can transform a relatively manageable problem into a serious outbreak within weeks.

Kenya’s rainfall is highly variable from one season and location to another, so farmers should not assume that every El Niño or above-normal rainfall season will affect every county in exactly the same way. The Kenya Meteorological Department’s seasonal forecasts emphasize regional differences in rainfall and encourage farmers to use weather information when planning agricultural activities.

For fruit farmers, the practical response is to prepare before disease becomes widespread. Good drainage, sanitation, canopy management, healthy planting material, regular scouting and appropriately timed crop protection can significantly reduce the conditions that allow diseases to take hold.

Why Excessive Rainfall Creates a Disease Problem

Most fruit-tree diseases do not develop simply because the tree gets wet once.

Disease development depends on the interaction between the pathogen, the host plant and the environment. This is often described as the disease triangle.

The pathogen must be present, the fruit tree must be susceptible, and environmental conditions must allow infection to occur.

During a prolonged wet season, the environmental component of this triangle becomes particularly favorable for many pathogens.

Rainfall can increase the duration for which leaves, flowers and fruit remain wet. It can also move spores between plant parts and from one tree to another. Where the orchard has poor airflow, the moisture may remain trapped within the canopy even after rainfall stops.

This is why a disease that was relatively insignificant during a dry period can become much more aggressive once persistent rains begin.

Which Fruit Diseases Become More Important During Wet Seasons?

The exact diseases depend on the crop, location and pathogen, but several important fruit diseases are favored by prolonged moisture.

Anthracnose is particularly important in mango and other fruit crops. KALRO identifies mango anthracnose, caused by Colletotrichum gloeosporioides, as a major disease in Kenya, with flowers, leaves, young fruit and shoots all susceptible. Wet and humid conditions favor infection, while rain splash helps spread spores.

Phytophthora-related diseases are another concern because prolonged soil saturation can create conditions favorable to root and crown rot. Trees affected by root and crown diseases may wilt even when the soil appears sufficiently wet because damaged roots cannot function normally.

Other diseases, including some bacterial spots, fruit rots, blossom infections and leaf diseases, can also become more problematic when moisture remains high.

This does not mean every wet-season disease should automatically be treated with fungicides. Diagnosis remains essential.

Anthracnose Can Become Particularly Destructive

Mango farmers should pay particular attention to anthracnose during prolonged wet weather.

The disease can attack leaves, flowers, young fruit and branches. On flowers, infection can kill individual flowers or entire flower stalks, reducing fruit set. On fruit, infections may remain inconspicuous while the fruit is immature and become much more obvious as it ripens.

This creates an important commercial problem.

A mango may appear healthy when harvested but develop significant anthracnose decay during ripening or after reaching the market. KALRO notes that post-harvest anthracnose can cause severe losses, particularly under humid conditions.

Consequently, managing anthracnose during an El Niño-type wet season is not only about protecting leaves. It is also about protecting the future marketability and shelf life of the fruit.

Rain Splash Is One of the Most Important Ways Disease Spreads

When rain strikes infected leaves, fruit or soil, it can dislodge spores or bacteria and carry them onto nearby plant tissues.

This is especially important for pathogens that depend on water for dispersal.

KALRO’s mango information explains that anthracnose spores are spread principally by splashing water, making rainy conditions during flowering and early fruit development particularly important for disease development.

This explains why disease can appear to move rapidly through an orchard after repeated rainfall.

A dense canopy can make the problem worse because leaves and fruit are close together and remain wet for longer.

Why Flowering Trees Are Especially Vulnerable

Flowering can be one of the most important disease-management periods in fruit production.

Flowers are delicate tissues and can be infected by pathogens that subsequently affect fruit set. In mango, for example, anthracnose can attack flowers and cause blossom blight.

A farmer may therefore see a healthy-looking tree with a large number of flower panicles but end up with very few fruits because infection occurred during flowering.

This is why disease scouting should begin before symptoms become obvious on mature fruit.

If a wet season coincides with flowering, the disease-management strategy may need to become more intensive, always within the requirements of the crop-protection products registered for the crop and disease.

Young Fruit Also Needs Protection

Young fruit can be highly susceptible to infection.

In mango, anthracnose infections can occur on immature fruit and later remain latent until ripening. This means that disease control cannot simply begin when harvest is approaching.

By that stage, some infections may already be established inside apparently healthy fruit.

KALRO’s mango guidance identifies bud formation, flowering and early fruit development as particularly susceptible stages for anthracnose.

The practical implication is important: wet-season disease management must be timed around crop development, not simply around the calendar.

Waterlogged Soil Creates a Different Kind of Disease Risk

Foliar diseases receive much of the attention during rainy seasons, but excessive rainfall can also damage the root system.

When soil remains saturated, the spaces normally occupied by air become filled with water. Roots then have less access to oxygen.

Prolonged saturation can weaken roots and create favorable conditions for pathogens such as Phytophthora species.

Affected trees may show yellowing, reduced growth, leaf drop and wilting. In severe cases, the farmer may see a tree wilting despite the soil being wet.

That apparent contradiction is a useful diagnostic clue.

A tree suffering ordinary drought stress may recover after receiving water. A tree whose roots have been severely damaged by waterlogging or root disease may continue declining despite abundant soil moisture.

Improve Orchard Drainage Before Heavy Rains Arrive

Drainage should be one of the first priorities when preparing an orchard for an unusually wet season.

Inspect the field for areas where water accumulates after rainfall. Low-lying sections, compacted soils and areas blocked by weeds or debris deserve particular attention.

Where appropriate, open or maintain drainage channels so excess water can leave the orchard. Avoid directing large quantities of runoff toward tree trunks.

For newly established orchards, planting on suitable raised positions or mounds can improve drainage in some soil types and production systems.

The exact approach depends on soil texture, slope and crop.

The goal is not to remove all water from the farm. Fruit trees need water. The objective is to prevent prolonged saturation of the root zone.

Never Allow Water to Stand Around the Tree Trunk

The area around the trunk, particularly the root collar, deserves special attention.

Do not create basins that continuously hold water against the trunk during a period of heavy rainfall.

Persistent moisture around the collar can increase the risk of crown and collar diseases in susceptible crops.

Keep drainage pathways open and avoid piling soil, wet organic material or other debris directly against the trunk.

This is particularly important in avocado and other fruit crops where root and collar diseases can seriously affect tree longevity.

Pruning Becomes More Important During Wet Weather

A dense canopy creates its own humid microclimate.

Leaves shade one another, airflow is reduced and water can remain trapped within the canopy for longer periods.

Appropriate pruning can improve air movement and reduce the time that foliage remains wet.

KALRO’s mango recommendations include regular pruning to improve ventilation and removal of diseased twigs and branches as part of disease management.

However, farmers should not respond to heavy rainfall by aggressively cutting the canopy.

Excessive pruning can expose branches and fruit to intense sunlight, reduce productive leaf area and create unnecessary wounds.

The objective is selective canopy management rather than severe tree reduction.

Remove Sources of Infection From the Orchard

Dead branches, infected fruit, diseased leaves and fallen plant material can serve as sources of inoculum for some pathogens.

During a wet season, leaving these materials in the orchard can provide pathogens with suitable conditions for producing new spores.

KALRO recommends removing infected mango fruits, leaves and twigs and maintaining field sanitation as part of anthracnose management.

Where infected branches are removed, do not simply leave them beneath the tree.

Depending on the disease and local recommendations, diseased material may need to be removed from the production area and appropriately destroyed or disposed of.

Do Not Prune Infected Trees and Then Spread the Disease

Pruning tools can move pathogens between trees.

If a farmer cuts an infected branch and immediately cuts healthy tissue with the same contaminated tool, disease organisms may be mechanically transferred.

This is particularly important when removing diseased material during wet conditions.

Use appropriate tool sanitation practices and avoid unnecessary cutting.

Where a serious disease is suspected, farmers should obtain crop-specific guidance on tool disinfection because the appropriate disinfectant, concentration, contact time and handling procedure matter.

Wet Conditions Can Make Fungicide Timing More Important

During prolonged rainfall, fungicide applications may become more complicated.

The farmer has to consider disease pressure, product characteristics, rainfall forecasts, crop stage, spray coverage and the label’s requirements.

Applying a fungicide immediately before heavy rain may result in reduced protection if the product has not had sufficient time to dry or become rainfast.

On the other hand, waiting until disease is already widespread can mean that much of the crop is already infected.

This is why disease management should be based on weather monitoring and disease scouting, rather than spraying at arbitrary intervals.

Do Not Simply Increase the Fungicide Dose

Heavy rain does not justify mixing a stronger fungicide concentration than the label allows.

Using excessive rates can cause crop injury, increase pesticide residues and environmental exposure, raise production costs and contribute to resistance problems.

The correct response to high disease pressure is better disease management—not automatically a stronger chemical dose.

Where fungicides are needed, farmers should use products registered for the particular crop and disease in Kenya and follow the label.

Rotate Fungicides According to Mode of Action

Wet seasons may tempt farmers to spray more frequently, which increases the importance of resistance management.

Repeatedly using the same fungicide or fungicide mode of action can select for less-sensitive pathogen populations.

Farmers should therefore rotate fungicides according to their FRAC mode-of-action groups, not merely alternate commercial brand names.

A different brand containing an active ingredient from the same resistance group may not provide the diversity the farmer expects.

This is particularly important when disease pressure remains high for several weeks.

Wet Weather Does Not Mean “Spray Every Week”

There is no universal weekly fungicide schedule for fruit trees.

Disease risk depends on the crop, pathogen, weather, plant growth stage, susceptibility of the variety and previous infections.

KALRO’s mango recommendations, for example, emphasize monitoring, sanitation, pruning and appropriately timed disease-control measures around vulnerable growth stages rather than treating the entire season as one continuous spraying period.

Farmers should therefore use the registered product label and crop-specific recommendations when determining application intervals.

Monitor the Orchard More Frequently

During a high-rainfall period, weekly or even more frequent scouting may be appropriate for susceptible crops and diseases.

Walk through different sections of the orchard rather than inspecting only trees along the farm road.

Look at new leaves, flowers, young fruit, mature fruit, twigs and the trunk.

Record the location and severity of symptoms. If disease repeatedly begins in one low-lying section, that pattern may point toward a drainage problem rather than simply indicating a need for more fungicide.

Learn to Recognize the Early Warning Signs

Early disease symptoms vary by crop, but farmers should pay attention to:

• small dark lesions on leaves;
• blossom browning or blackening;
• premature flower drop;
• dark spots on young fruit;
• sunken fruit lesions;
• unusual leaf drop;
• twig dieback;
• gumming or bark lesions;
• water-soaked tissue;
• root or collar discoloration;
• sudden decline in poorly drained areas.

The earlier these symptoms are recognized, the more options the farmer generally has.

KALRO emphasizes early identification and control of major passion fruit diseases because severe disease can cause very large yield losses.

Pay Particular Attention to Passion Fruit

Passion fruit can be highly vulnerable to disease problems, and prolonged wet conditions can create favorable environments for several pathogens.

KALRO identifies diseases as one of the major constraints to passion fruit production in Kenya and emphasizes early identification and control.

Farmers should monitor leaves, stems and fruit closely and avoid allowing excessive canopy humidity.

Because passion fruit is commonly grown on trellises, canopy structure can have a major influence on ventilation.

Diseased vines should be managed promptly according to the specific disease rather than simply increasing pesticide applications.

Avocado Farmers Should Watch the Root Zone

Avocado requires particular attention during periods of excessive rainfall because poorly drained soil can create serious root-health problems.

Phytophthora root rot is associated with prolonged soil moisture and poor drainage. Trees may show reduced vigor, yellowing, canopy decline and wilting even when the soil contains plenty of water.

The most effective strategy is therefore preventive.

Choose well-drained sites, avoid prolonged waterlogging and maintain healthy root systems. Do not assume that a declining avocado tree needs more irrigation simply because its leaves are wilting.

Citrus Farmers Should Watch for Anthracnose and Fruit Rots

Citrus anthracnose is associated with wet conditions and can cause twig dieback, premature leaf drop, fruit staining and post-harvest decay.

Spores can be moved from infected leaves and wood to fruit by dripping and splashing water.

This makes sanitation particularly important.

Dead twigs and branches should be removed during suitable dry conditions, because these tissues can serve as sources of fungal inoculum.

Farmers should also pay attention to fruit handling because infections established in the orchard may become much more obvious after harvest.

Mango Farmers Should Protect the Flowering Period

For mango producers, flowering and early fruit development can be especially important during wet conditions.

Rainy weather during flowering can increase anthracnose infection of flowers and reduce fruit set.

Farmers should therefore scout flower panicles closely during prolonged wet periods.

Avoid allowing dead and diseased material to accumulate in the canopy, maintain ventilation and use appropriate registered disease-control measures where necessary.

The exact fungicide program should be based on current Kenyan product registration, the disease diagnosed and label directions.

Do Not Confuse Disease With Nutrient Deficiency

Wet seasons can also change nutrient availability and root function.

A tree may develop yellow leaves because its roots are oxygen-starved or diseased, not because the soil simply lacks nitrogen.

Adding fertilizer without identifying the cause can waste money and potentially worsen the situation.

For example, a tree with damaged roots may be unable to absorb nutrients efficiently even when those nutrients are present in the soil.

Before applying corrective fertilizer, examine soil moisture, drainage, root health and the distribution of symptoms across the orchard.

Heavy Rain Can Also Increase Fruit Cracking and Secondary Rot

Fruit cracking and disease can interact.

Rapid changes in water availability can cause some fruits to crack, creating openings through which fungi and bacteria can enter.

Once the skin is broken, the exposed tissues are much more vulnerable to decay.

Therefore, farmers dealing with fruit cracking during a wet season should remove severely damaged fruit, maintain sanitation and monitor the cracks for secondary infection.

Water management remains important, particularly during periods when dry conditions alternate with intense rainfall.

Avoid Working in the Orchard When It Is Extremely Wet

Field operations themselves can contribute to disease and soil problems.

Walking or driving heavy equipment across saturated soil can cause compaction. Compacted soil reduces pore space and can interfere with root aeration and drainage.

Moving from infected trees to healthy trees while handling wet plant material can also contribute to disease spread in some systems.

Where practical, allow excessively wet fields to drain before carrying out unnecessary operations.

This is especially important in orchards established on heavy soils.

Use Weather Forecasts as a Disease-Management Tool

Farmers should not treat weather information as something relevant only to planting dates.

Rainfall forecasts can help determine when disease risk is likely to increase.

The Kenya Meteorological Department provides seasonal and shorter-term agricultural weather information, including rainfall outlooks and advisories. Its 2026 Agromet bulletins, for example, have highlighted above-normal rainfall in some regions and advised farmers to maintain drainage to prevent waterlogging and root rot.

Farmers can combine these forecasts with field observations.

If a disease-prone crop is approaching flowering and a period of prolonged rainfall is forecast, that is a reason to intensify scouting and prepare the orchard—not necessarily a reason to start indiscriminately spraying.

What Should Farmers Do Before an El Niño Season?

Preparation should begin before the first major rains.

Clear blocked drainage channels and identify places where water normally accumulates. Remove dead branches and old diseased fruit where appropriate. Inspect trees for existing infections and deal with them before wet conditions make disease spread easier.

Check the condition of pruning equipment and irrigation systems.

Review the fungicides available on the farm and confirm that they are registered for the crops and diseases you expect to manage. Check their active ingredients and FRAC groups so that you do not accidentally rely on one mode of action throughout the season.

Most importantly, establish a monitoring routine.

What If the Heavy Rains Have Already Started?

Do not panic and start spraying everything.

First assess the orchard.

Identify which diseases are actually present, where they are occurring and which growth stages the trees are in. Check whether drainage is adequate and whether the canopy is excessively dense.

Remove manageable sources of infection where appropriate and improve airflow.

Then determine whether a registered crop-protection intervention is justified.

If the disease is already severe, consult an extension officer, agronomist or plant-health specialist rather than repeatedly applying products without a diagnosis.

What If a Tree Starts Wilting While the Soil Is Wet?

Investigate the root zone.

This symptom can be particularly important in areas with poor drainage.

Carefully examine the root collar and, where appropriate, roots for discoloration, decay or other abnormalities. Look at whether other trees in the same poorly drained area are affected.

If only trees in low-lying sections are declining, waterlogging or a soil-borne disease becomes more plausible.

Do not respond automatically by adding more irrigation.

Should Farmers Stop Fertilizing During Heavy Rains?

Not automatically.

Fruit trees still require nutrients during wet periods, but fertilizer decisions should consider crop stage, soil fertility, rainfall and root condition.

If roots are severely waterlogged, nutrient uptake may be impaired.

Applying large quantities of fertilizer immediately before or during very heavy rainfall can also increase nutrient losses through leaching or runoff.

Use soil-test information where available and follow crop-specific nutrient recommendations rather than applying fertilizer simply because the trees appear pale.

The Role of Disease-Resistant Varieties

Variety selection is one of the strongest long-term tools available to farmers.

Where suitable resistant or tolerant cultivars exist, they can reduce disease pressure and potentially reduce dependence on fungicides.

However, resistance is normally specific to particular diseases and may not provide complete protection.

KALRO’s mango recommendations, for example, include growing varieties that are less susceptible to anthracnose as one component of disease management.

When establishing a new orchard, farmers should therefore consider disease resistance alongside yield, fruit quality, climate suitability, maturity period and market requirements.

The Commercial Cost of Poor Wet-Season Disease Management

Disease during an El Niño-type season can affect much more than yield.

Fruit quality may deteriorate. More fruit may be rejected by buyers. Harvesting and sorting costs can increase. Post-harvest rots can shorten shelf life, while additional crop-protection applications can increase production costs.

For export-oriented farmers, disease damage can be especially serious because appearance, quality and shelf life are important components of market acceptance.

Mango anthracnose illustrates this clearly: KALRO identifies anthracnose and stem-end rot as major post-harvest diseases capable of causing substantial losses and reducing mango shelf life in Kenya.

This means disease prevention should be viewed as an investment in marketable yield, not merely an attempt to keep leaves looking healthy.

A Practical Wet-Season Disease Management Plan

A farmer preparing for an unusually wet season can organize the orchard around five priorities.

First, manage water. Keep drainage systems functional and prevent prolonged waterlogging.

Second, reduce inoculum. Remove appropriate sources of infection, including diseased fruit, leaves and branches.

Third, improve the canopy. Maintain adequate airflow without excessive pruning.

Fourth, monitor frequently. Pay particular attention to flowering, fruit set and young fruit.

Fifth, use crop protection strategically. Apply only appropriate, registered products, follow label instructions and rotate fungicide modes of action where chemical control is necessary.

These measures reinforce one another.

A fungicide program cannot compensate for an orchard that is permanently waterlogged. Likewise, good drainage cannot prevent every foliar disease if infected material is allowed to accumulate throughout the canopy.

The Most Important Mistakes to Avoid

One of the biggest mistakes is waiting until disease is severe before responding.

Another is spraying automatically whenever it rains, without identifying the disease or considering whether the product is appropriate.

Farmers should also avoid increasing pesticide concentrations because disease pressure is high.

Do not ignore drainage problems while concentrating entirely on leaves and fruit.

Avoid excessive pruning during wet conditions simply to “open the tree,” particularly when the resulting wounds may become infection sites.

Finally, do not assume that a healthy-looking fruit is necessarily free from disease. Some infections, particularly anthracnose, can remain latent and become visible during ripening.

El Niño Management Should Continue After the Rains

Disease management does not end when the heavy rainfall stops.

Inspect trees for infections that developed during the wet period. Remove appropriate dead or diseased plant material during suitable weather. Monitor fruit during ripening and pay close attention to post-harvest disease.

Review which areas of the farm suffered the greatest damage.

If disease repeatedly occurred in the same low-lying area, drainage improvements may be necessary before the next rainy season.

If one variety performed significantly better than another, disease response should become part of future planting decisions.

Every wet season should therefore provide information that improves the next production cycle.

Final Takeaway

An El Niño season does not automatically mean that fruit trees will suffer severe disease. The real risk comes from the combination of high moisture, prolonged leaf wetness, rain splash, saturated soils, dense canopies, susceptible plant material and existing sources of infection.

Kenyan farmers can reduce this risk by preparing before heavy rains, maintaining effective drainage, removing disease sources, improving canopy ventilation, using healthy planting material, scouting regularly and responding to disease at the correct crop stage.

Chemical control still has a place, but it should be used strategically. Fungicides should be selected for the disease diagnosed, applied according to their Kenyan label requirements and rotated by mode of action rather than simply by brand.

Above all, farmers should use weather information as part of orchard management. The Kenya Meteorological Department’s forecasts can help farmers anticipate periods of elevated rainfall and adjust monitoring and field preparation accordingly.

The best wet-season disease strategy is therefore not a bigger spray program. It is a better-managed orchard in which water, canopy, sanitation, planting material, disease monitoring and crop protection work together.

Farmers seeking certified seedlings and expert guidance can contact Organic Farm via:

Website: www.organicfarm.co.ke

Call or WhatsApp: +254 712 075 915

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