Algal leaf & fruit spot
Rusty, velvety green-grey spots that downgrade fruit appearance in shaded, humid orchards — caused by an alga, not a fungus, and controlled mainly by opening up the canopy.
- Cause
- Parasitic alga (Cephaleuros virescens)
- Favoured by
- Shade and humidity
- Main damage
- Downgrades fruit appearance
- Best fix
- Prune for light and airflow
Overview
Algal leaf and fruit spot is unusual among plant diseases because the organism causing it, Cephaleuros virescens, is a parasitic alga rather than a fungus or bacterium — a simple, plant-like living thing that normally grows on damp surfaces but can also grow directly on leaves, twigs and fruit. It forms rusty, velvety, green-grey raised spots that make affected fruit look dirty and lower its market grade.
The alga thrives in shade and humidity, so it is worst in dense, poorly-pruned orchards where light and air do not reach the inner canopy. Unlike many fungal diseases it does not usually rot the fruit outright — the main damage is cosmetic, downgrading fruit that would otherwise sell well. Because shade and stagnant humidity are the underlying cause, opening up the canopy through pruning does more long-term good than any single spray.
How to identify it
- Rusty, velvety, slightly raised green-grey spots on leaves and fruit.
- Spots are most common on the shaded, inner or lower parts of the canopy where humidity stays high.
- Affected fruit looks blemished and dirty even though the flesh underneath is usually still sound — the main loss is a lower grade at sale, not a rotten fruit.
- Orchards that are dense, poorly pruned or heavily shaded show far more spotting than open, well-aired ones.
What causes it and when
- A parasitic alga, Cephaleuros virescens — not a fungus — that grows directly on plant surfaces in favourable conditions.
- Shade and high humidity are what it needs to establish and spread; it is rarely a problem in open, sunny, airy orchards.
- A dense, overgrown canopy or trees planted too close together keep humidity high around the leaves and fruit for longer each day, which favours the alga.
- It tends to build up over several humid seasons in an orchard that is not regularly pruned.
Cultural & organic control
- Prune to open up the canopy and let more light and air reach the inner branches and fruit — this is the single most effective long-term control.
- Space trees adequately at planting so mature canopies do not crowd each other and trap humidity.
- Improve drainage and avoid overhead irrigation that wets the canopy unnecessarily, since standing moisture favours the alga.
- Remove badly-spotted twigs where practical to reduce the amount of alga carried over to the next season.
Chemical control
- A recommended copper-based spray helps check the alga on badly affected trees, applied alongside pruning rather than instead of it.
- Since the spots are mainly cosmetic, treatment is usually only worthwhile where the crop is being grown for a market that pays for clean-looking fruit.
- Confirm the current recommended copper product and dose for your area with your local Krishi Vigyan Kendra (KVK).
Prevent it coming back
- Keep up an annual pruning routine that maintains an open canopy with good light and airflow — this addresses the root cause, shade and stagnant humidity.
- Avoid overcrowded planting and unnecessary overhead wetting of the canopy.
- A copper spray on a bad year can help, but pruning discipline is what keeps the problem from coming back every season.
Crops most at risk
Where this problem is called out in our crop guides — open a crop for the full package of practices.
Frequently asked questions
Is algal leaf spot a fungus, and does the same fungicide work on it?
No — it is caused by a parasitic alga (a simple, plant-like organism), not a fungus, though it can look similar to a fungal leaf spot at first glance. A copper-based spray does help check it, similar to many fungal sprays, but the bigger and more lasting fix is pruning to remove the shade and humidity the alga needs, since that addresses the actual cause.
Does algal spot actually rot my guava fruit, or just mark it?
Mostly just marks it. The rusty, velvety spots make the fruit look dirty and lower its market grade, but the flesh underneath is usually still sound, unlike a true fruit rot. The main cost is a lower sale price for blemished fruit, not fruit loss.
Why does pruning help against algal spot more than spraying does?
Because the alga needs shade and stagnant humidity to establish and spread, and a dense, unpruned canopy provides exactly that around the inner branches and fruit. Opening up the canopy with regular pruning lets in light and lets air move through, which removes the conditions the alga depends on — a copper spray helps in a bad year, but it does not fix a permanently shaded, humid canopy the way pruning does.
Symptoms and control measures follow ICAR, KVK and state agricultural-university package-of-practices; any named chemical is a registered use. Always read the product label and confirm the dose for your crop and zone. editorial policy.