Prevention of rust disease in coffee

Hemileia vastatrix is the most dangerous disease in the world's coffee, causing 30-70% yield loss when it breaks out. Instructions for early identification, seasonal spraying schedule and disease outbreak treatment for Robusta orchards in the Central Highlands.

Summary: Rust once caused Sri Lankan coffee to die out in the 1880s. In Vietnam, rust is still a common disease on Robusta coffee, especially in the high Lam Dong and Dak Lak regions with lots of dew. Unlike rice blast that spreads quickly in a few days, rust develops silently over weeks — by the time it becomes apparent, it has already spread widely. This article focuses on early recognition and seasonal spraying schedules.

Applies to: Robusta Coffee in Dak Lak, Lam Dong, Gia Lai, Dak Nong, Kon Tum.
Duration: Room all year round. Pressure is strongest in June-October (rainy season).
Difficulty level: Medium.
Estimated additional costs: 2-4 million VND per hectare per year for periodic rooms.

Characteristics of rust fungi

*Hemileia vastatrix* is a fungus that specializes in parasitic coffee. Features:

Spores spread through air and wind. This orchard has an epidemic that can spread to neighboring orchards within a few days when the wind is favorable.
Grows strongly when the temperature is 22-28 degrees Celsius + humidity above 90 percent. This is the condition of the rainy season in the Central Highlands.
Attack leaves — causes mass leaf loss when severe.
Indirect consequences: trees lose leaves = reduced photosynthesis = small fruit, light seeds, yield reduced by 30-70 percent.
Spores sleep on fallen leaves + dry branches, surviving 6-12 months. The orchard once had an unsanitary epidemic = outbreak again next season.

Other than the quick death of pepper or rice blast, rust does not kill the plant immediately but causes the orchard to gradually decline. Many orchard owners ignore it because the tree is still "green", but the yield gradually decreases with each crop.

Symptoms and severity

Early symptoms

Small round spot 2-5 millimeters on the underside of the leaf, light yellow.
The center of the spot turns a characteristic orange-yellow color — similar to rust (this is the name).
The upper surface of the leaf has corresponding pale yellow spots.
When you touch the underside of the leaves, you see fine powder — that's mushroom spores.

Symptoms develop

The spots gradually get bigger and spread. One leaf can have 5-15 spots.
The leaves gradually turn yellow from the edges to the inside.
The spots turn dark brown as the leaves age.
Leafs fall — first the lower leaves, then the middle leaves.

Level and assessment

Count the average number of spots on 30 leaves on 5 random plants in the orchard:

Level	Average number of spots per leaf	Action
Light	1-2 spots	Monitor, no need to spray yet
Average	3-7 spots	Spray immediately
Heavy	More than 7 spots or 20-30 percent leaf loss	Spraying strongly + improving the orchard

Outbreak conditions

Rust explodes strongly when present at the same time:

Altitude areas above 600 meters — Lam Dong and Dak Lak highlands have the highest pressure.
Rain lasting more than 3 days + morning dew.
Dense tree canopy + thick planted orchard — increases local humidity.
Weak tree — after severe drought, after other diseases, lack of nutrition.
The orchard had an epidemic last year and was not cleaned.

Seasonal Room

At the end of the dry season, March-April

Cut diseased and dry branches. Take it away from the orchard to burn.
Rake up fallen leaves under the tree — spores lie dormant on fallen leaves.
Recovery fertilizer: organic + potassium + micronutrients (Cu, Zn). Copper (Cu) is especially important — copper has a natural antifungal effect.
Spray the background once with *Copper oxychloride* (copper-based fungicide) or *Mancozeb* (manganese-zinc-based fungicide) before the first rain of the season.

The beginning of the rainy season in May-June

Spray once with *Triadimefon* or *Tebuconazole* (triazole-based fungicide) when the first rain appears.
Check weekly — especially trees in thick canopy layers, near orchard edges.

The rainy season peaks in July-September

This is the most intense stage. Need to check weekly:

Count the spots on the middle leaves.
Observe the underside of the leaves — orange spore powder.
When the threshold of 3 spots/leaf is exceeded, spray immediately.

Spray every 14-21 days while fluid pressure is still present. Rotate active ingredients between doses — *Triadimefon*, *Tebuconazole*, *Azoxystrobin*, *Difenoconazole*.

Late rainy season October-November

Spray broad spectrum once at the end of the crop.
Clean up the orchard — prepare for the dry season.

Spray correctly

Suitable active ingredients (rotate to avoid resistance)

Copper-based fungicides: *Copper oxychloride*, *Copper hydroxide*. Good room, little resistance. Suitable for the end of the dry season and the beginning of the rainy season.

Triazole group of fungicides: *Tebuconazole*, *Triadimefon*, *Hexaconazole*. Good treatment, long lasting treatment.

Strobilurin-based fungicides: *Azoxystrobin*, *Trifloxystrobin*. Good prevention and treatment, little harm to natural enemies.

Combination of 2 active ingredients: *Tebuconazole + Trifloxystrobin*, *Mancozeb + Tebuconazole*. High efficiency + low resistance.

Spraying technique

Spray in the cool afternoon from 16-18 hours.
Spray both sides of leaves — most important, especially the underside (where the spores are).
The tank pressure is strong enough to spray high canopy.
Amount of water 500-800 liters per hectare — enough to cover evenly.
Do not spray before heavy rain in the next 4-6 hours (the treatment will be washed away).

Improve orchard conditions

Spraying is only a temporary solution. Improve orchards to reduce long-term pressure:

Prune branches for ventilation — thick canopy is the ideal environment for rust.
Weed management — dense grass increases local moisture.
Balanced nitrogen-potassium fertilization — high nitrogen creates soft leaves that are susceptible to infection.
Auxiliary plants provide adequate shade — too much shade = high humidity = strong rust.

Treat disease outbreaks

When the disease has clearly broken out in the orchard area:

Step 1: isolate the diseased area, limit movement to avoid spreading spores through clothing.

Step 2: cut off diseased leaves and overly infected branches. Take it away from the orchard to burn, do not leave it in the orchard.

Step 3: spray according to regimen:

First time: high dose of triazole-based active ingredient.
Second time after 14 days: original active ingredient strobilurin.
3rd time after 14 days: combine 2 active ingredients from different groups.

Step 4: organic fertilizer + potassium supplement for plant recovery. Trees that have lost a lot of leaves need nutrients to restore growth.

Monitor every 7-14 days during the rainy season

[ ] Count spots on leaves on 5 random plants (30 leaves each).
[ ] Observe the underside of the middle leaf — orange spores.
[ ] Rate of fallen leaves compared to before.
[ ] Condition of the back of new leaves — whether infected early or not.
[ ] Weather — rain, dew, humidity.

Common mistakes

Take it lightly because the tree is still "green": rust reduces productivity implicitly, not clearly visible in a single season.

Spray only on the upper surface of the leaves: spores on the lower surface. Both sides must be sprayed.

Using the same active ingredient multiple times: fungus becomes resistant quickly. Rotate 3-4 active ingredients.

Abandon cleaning after crop: spores lie dormant on fallen leaves, causing outbreaks next crop.

High nitrogen fertilizer in the rainy season: soft leaves are susceptible to infection, making the disease worse.

Take notes

Prophylactic spraying session — date, active ingredient, dose.
Outbreak — date, extent, orchard area.
Rate of leaves falling at the end of the season.
Productivity compared to previous year.

References

*Prevention of coffee diseases* — Central Highlands Agricultural and Forestry Science and Technology Institute (abbreviated name WASI), 2022.
*Coffee rust prevention handbook* — Plant Protection Department, 2021.