Instructions for preventing red algae disease (spotted algae) for pepper

Red algae or seaweed spots often grow strongly in damp pepper orchards, dense canopy, and poor drainage. Instructions for identifying red-brown spots on leaves/fruits, cleaning the orchard, pruning for ventilation and spraying copper-based crop-protection products at the right time.

Summary: Red algae disease, also known as seaweed spot, does not kill pepper as quickly as *Phytophthora*, but reduces photosynthesis, deteriorates fruit and affects the quality of dried pepper. The disease often appears in dense, humid, and poorly ventilated orchards. This article shows how to recognize and distinguish from anthracnose and room by orchard hygiene + copper-based treatment.

Applies to: Pepper orchards for business in the Central Highlands and Southeast, especially orchards with dense canopy and prolonged humidity during the rainy season.
Duration: Room before and during the rainy season; Spray the orchard after harvest if the orchard has been seriously damaged.
Difficulty level: Basic. It is important to prune well and treat it early.
Estimated additional costs: 1-2.5 million VND per hectare per year for orchard cleaning and room spraying.

What is Red Algae

Red Algae Disease is caused by a group of parasitic algae, commonly recognized as *Cephaleuros virescens*. This agent grows on leaves and green fruits and sometimes spreads on stems/branches when conditions are too humid.

Features:

Appears a lot during the rainy season.
Prefers dense, moist, poorly drained orchards.
Causes reddish brown and gray brown spots on leaves and fruit.
Reduces photosynthesis if spots spread into patches.
Deforms pepper after harvest if it causes damage to the fruit.

This disease usually does not flare up violently within a few days, but if left for a long time, it will cause the tree to decline and reduce the quality of the shipment.

Symptoms on leaves and fruit

On leaves

Small spots 1-5 millimeters, gray brown, reddish brown or brick red.
The spots are usually located on the upper surface of the leaves.
When the spots get old, the surface is slightly rough or has a smooth velvet layer.
Many spots linked into arrays.
The leaves suffer from severe photosynthesis, turn yellow and fall prematurely.

The spots are often more obvious on the leaf side facing the light, especially the leaf layer near the base in humid orchards.

On green fruit

There are reddish brown spots on the green left side.
Orchid spots make fruit tissue dark and ugly.
When drying, pepper can deform, peel off the skin or leave dark spots.
With white pepper, dark spots reduce the commercial value more clearly.

White pepper orchards need to pay attention to this disease because the color of the seeds after processing is a selling price factor.

Distinguished from anthracnose

Red algae and anthracnose both create spots on leaves/fruits, but there are a few differences:

Sign	Red algae/spotted algae	Anthracnose
Spot color	Red brown, brick red, slightly rough	Dark brown, concave, with yellow halo
Location	The top side of the leaf, the left side facing the light	Leaves, branches, stems, fruit; spreads strongly when it rains
Surface	Maybe a little velvet	Often concave, clearly dead tissue
Condition	Dense, humid orchard, weak growth	Lots of rain, wounds, dampness

If you are not sure, handle in the general direction: clean the orchard, prune for ventilation, spray the room with copper group or appropriate treatment according to the label.

Why does the disease thrive

Favorable conditions:

The dispersion is too thick, little light gets in.
The shade trees are too dense.
Ditches have poor drainage, soil stays wet for a long time.
Leaf branches close to the base should not be pruned.
Dried leaves/fruits left on the tree and at the base.
The tree fails due to lack of nutrition and weak roots.

Red algae is not just a “leaf disease”. It reflects that the orchard is too humid and lacks ventilation.

Prevention by farming

Prune for ventilation

Cut off the eel vines and low branches close to the ground.
Prune out useless branches in the canopy.
Adjust shade trees, avoid blocking more than 50-60 percent of the light.
After harvesting, prune to restore the orchard to dry faster after rain.

A well-ventilated orchard reduces the pressure of red algae, anthracnose and many other fungal diseases at the same time.

Cleaning disease sources

Collect diseased leaves, dried fruits, and remaining diseased stems.
Do not let disease residue pile up under the tree.
Clear the grass around the base just enough to allow ventilation, without making the soil completely bare.
Disinfect tools when pruning seriously diseased branches.

Algae can survive on dried leaves/fruits left in the orchard. Post-harvest hygiene is very important.

Improve drainage

Clean ditches before the rainy season.
Do not let water accumulate around the base of the pepper.
For heavy soil, open a sub-groove in the middle of the row.
Cover the roots with airy materials that do not cover the root collar.

Prolonged wet soil weakens plants and increases leaf and fruit diseases.

Spraying and treatment

When the illness is mild

If only a few leaves have spots:

Prune seriously diseased leaves/branches.
orchard cleaning.
Spray water to wash the canopy after harvest if there is a lot of dust and algae.
Follow up in 10-14 days.

There is no need to chemically spray the entire orchard if disease rates are low and the weather is dry.

When the disease spreads during the rainy season

Copper-based medications can be used according to the label:

Spray evenly on leaves, branches and fruit bunches.
Spray in the cool, dry afternoon.
Avoid spraying right before rain.
Repeat after 10-14 days if the disease still spreads and the rain persists.

Copper-based drugs are often effective against red algae, but must still be accompanied by pruning and drainage. Spraying when the orchard is still damp will easily cause the disease to return.

After harvest

The orchard has a lot of red algae, so the orchard should be sprayed after harvest:

Prune branches first.
Collect disease residue.
Copper based spray or anti-fungal/algae spray is allowed.
After 7-10 days of organic fertilizer + Trichoderma to restore the soil.

Seasonal Tracking

The beginning of the rainy season

Check the layer of leaves near the base.
Prune low and useless branches.
Clean ditches.

In the middle of the rainy season

Check for spots on green fruit.
Spray if the disease spreads quickly.
Keep shade trees at a moderate level.

After harvest

Recovery pruning.
Collect the stalks/dried fruit.
Spray the orchard if the previous crop was seriously ill.

Monitor periodically

[ ] Percentage of leaves with red algae spots.
[ ] Percentage of fruit clusters with spots.
[ ] Canopy ventilation level.
[ ] Drainage condition after rain.
[ ] Number of copper root sprays and spraying time.

If red algae appears repeatedly over many years, it is necessary to review the shading and drainage design, not just increase the number of spraying times.

Common mistakes

Mistaking red algae for malnutrition: adding fertilizer does not solve the disease source.

Only spray but do not prune the canopy: the orchard is still damp, the disease returns quickly.

Spray before rain: treatment will be washed away, low effectiveness.

Leave dried fruit and diseased leaves on the tree after harvest: keep the disease source for the next season.

Shading too thick: pepper is cooler but humidity is high, leaf and fruit diseases increase.

Take notes

Date of first spotting.
Location in the orchard: low plot, dense plot, plot near shade trees.
Ratio of diseased leaves/fruits.
Measures taken: pruning, cleaning, spraying.
Results after 10-14 days.

Batch recording helps to know which areas are poorly drained or over shaded.

References

*Pepper pests and prevention measures* — Vietnam Academy of Agricultural Sciences, National Agricultural Extension Center.
*Guidelines for preventing Red Algae disease (spotted algae)* — Hainong technical document.
*Managing pepper diseases during the rainy season* — Plant Protection Institute.