Leaf spot in citrus is not one disease but several, and the most damaging of them behaves in a way that catches growers out every year. Greasy spot, the main defoliating leaf disease of citrus, builds its inoculum in the dead leaves lying on the orchard floor. Spores released from that decomposing litter infect the new flush through the underside of the leaf, and then nothing happens for two or three months while the fungus develops invisibly inside the tissue. By the time the blisters and yellowing appear, the infection is months old and no spray will undo it. That is why the answer to citrus leaf spot begins with the ground under the tree and with a protective spray timed well before any symptom is visible.
Symptoms
Greasy spot
- Yellow, chlorotic mottling on the upper leaf surface, without any clear margin, often the first thing noticed
- Raised, blister-like swellings on the underside directly beneath the yellow patches, turning from yellow-brown to dark brown and finally black, with a distinctly greasy or oily appearance
- Lesions visible mainly on the lower surface, unlike most other citrus leaf diseases
- Heavy premature leaf fall, concentrated in the lower and inner canopy and worst in the cool months, leaving a thin tree going into the next season
- Small black speck-like pustules on the fruit rind in severe cases, which downgrade the produce without rotting it
Alternaria brown spot
- Brown to black necrotic spots on young leaves, each ringed by a bright yellow halo
- Dark, corky veins radiating outward from the lesion, which is the distinguishing feature of this disease
- Lesions on twigs and on young fruit, with heavy drop of affected fruit and shoot dieback
- Damage concentrated on tender flush, with mandarins and tangelos far more severely affected than other types
Anthracnose and scab
- Tan to greyish-brown flat dry patches with darker margins, usually on leaves of trees already weakened by stress, along with twig dieback and staining on the fruit
- Wart-like, raised, corky outgrowths on leaves in the case of scab, with the leaf becoming puckered and distorted around them
Causes
- Mycosphaerella citri, the greasy spot fungus, which produces its spores in decomposing fallen citrus leaves on the orchard floor and releases them onto the new flush with rain and high humidity
- A latent period of two to three months between infection and visible symptoms in greasy spot, which means the disease is always far more advanced than it appears and a curative spray is never an option
- Infection through the stomata on the underside of the leaf, so spray coverage of the lower surface decides whether protection works
- Alternaria alternata, the brown spot fungus, which overwinters in lesions on twigs and leaves and spreads by wind-blown spores onto tender flush during wet weather
- Colletotrichum gloeosporioides and Elsinoe fawcettii, causing anthracnose and scab respectively, both surviving on infected twigs and debris in the canopy
- Long periods of leaf wetness from monsoon rain, heavy dew or overhead irrigation, without which none of these fungi can infect
- Warm humid weather between roughly 25 and 30 degrees Celsius, which is when spore release and infection peak
- Dense, unpruned canopies with poor air movement, which hold humidity inside the tree and slow drying after rain
- Fallen leaf litter left to accumulate under the tree, which is the single biggest source of greasy spot inoculum in an orchard
- Repeated flushes of tender growth, which remain susceptible for two to three weeks each and give the fungi a continuous supply of vulnerable tissue
- Trees weakened by water stress, poor nutrition or waterlogging, which succumb more readily, particularly to anthracnose
How to Identify
Turn the leaf over. That single step separates the most important of these diseases from the rest. Greasy spot shows its raised, blistered, greasy-looking brown or black lesions on the lower surface, with only a vague yellow mottle above and no sharp margin. Nothing else in citrus produces that combination.
Alternaria brown spot is the opposite in character: a clearly defined necrotic brown-black spot with a bright yellow halo, and dark corky veins running outward from it like spokes. Those darkened veins are the confirming feature. Anthracnose gives flat, dry, tan-brown patches on trees that are visibly under stress, and scab produces raised wart-like corky outgrowths that distort the leaf.
The distinction that matters most in practice is from citrus canker, since the two are often confused and are treated differently. Canker is bacterial. Its lesions are raised and corky with a crater-like centre, ringed by a yellow halo, and the same lesion appears on both surfaces of the leaf. Greasy spot blisters sit on the underside only. Where the lesions line up along silvery serpentine tunnels, the problem is canker following leaf miner damage, not a fungal leaf spot.
Inspect the orchard weekly through the monsoon and with every new flush, checking the underside of leaves in the lower and inner canopy where greasy spot begins. Also look at the ground: a thick mat of old citrus leaves under the tree tells you the inoculum source is present regardless of what the canopy looks like today.
Control Methods
Chemical Control
1. Copper Oxychloride 50% WP
Copper is the standard protectant for citrus leaf spots. Copper ions released on the leaf surface disrupt enzyme systems in the fungal spore and stop it germinating, and because the action works at several sites at once, resistance develops very slowly even after many years of use. It is purely protective, so it must be on the leaf before the spores arrive rather than applied after the spots appear. Direct the spray upward into the canopy to wet the underside of the leaves, since that is where greasy spot infects and where a downward spray never reaches.
Product Recommendation
2. Copper Sulphate 47.15% + Mancozeb 30% WDG
This combination pairs two multisite protectants with different chemistry. The copper fraction works as above, while mancozeb, a dithiocarbamate, inactivates sulphydryl groups in fungal enzymes and broadens the spectrum, giving better cover against Alternaria brown spot and anthracnose than copper alone. The water-dispersible granule formulation mixes cleanly and sticks well through rain, and mancozeb also supplies trace manganese and zinc to the tree. Like copper, it is protective rather than curative and has to be applied ahead of infection.
Product Recommendation
Getting the Timing and the Coverage Right
Both products above are protectants, which makes timing the whole game. For greasy spot the infection period is the early wet season, and because symptoms take two to three months to appear, a spray put on when the blisters are visible protects nothing on that flush. Apply as the new flush hardens off at the start of the wet period, and repeat through the monsoon as fresh growth emerges. For Alternaria brown spot, cover each new flush while the leaves are still tender, since that is the window when they can be infected.
Coverage decides the rest. Greasy spot enters through the stomata on the lower leaf surface, so use a hollow cone nozzle, direct the lance upward into the canopy from below, and keep the water volume full so the spray reaches the inner and lower branches where the disease starts. A high-volume spray that wets the undersides of leaves in the lower canopy is worth more than two well-intentioned passes over the top of the tree.
Cultural Control
- Manage the leaf litter, which is the most important single measure against greasy spot; rake up and remove or compost fallen citrus leaves, or speed their breakdown so the fungus cannot complete its cycle in them
- Prune to open the canopy so air moves through the tree and foliage dries quickly after rain, which shortens the wetness period all these fungi depend on
- Remove and destroy dead twigs, cankered wood and infected shoots from the canopy, where anthracnose and scab overwinter
- Irrigate through drip or basin rather than overhead sprinklers, so the foliage is not wetted unnecessarily
- Keep trees in good vigour with balanced nutrition and steady irrigation, since stressed trees are markedly more susceptible, particularly to anthracnose
- Correct waterlogging and improve drainage, which weakens roots and predisposes the whole tree to disease
- Space trees to allow air movement between them rather than crowding the planting
- Manage nitrogen so flushes are fewer and more uniform, because scattered tender flush through the wet season gives the fungi a continuous supply of susceptible leaves
- Plant certified disease-free nursery stock, and avoid bringing infected material into a clean block
- Keep the orchard floor free of weeds, which add to humidity within the lower canopy
Orchard Sanitation
Most of the physical work against citrus leaf spot is sanitation, and it costs little beyond labour. Clear the fallen leaves from under the trees, especially before the wet season begins, since that litter is where greasy spot spores are produced. Where removal is impractical across a large block, encourage rapid decomposition by keeping the litter moist and worked into the soil, so the fungus loses the intact leaf tissue it needs.
Alongside that, prune out dead and diseased twigs during the dry season and burn them rather than leaving them in the orchard, sterilise pruning tools between trees, and clear fallen fruit. Remove water shoots and suckers, which push out soft flush at odd times and give the fungi susceptible tissue outside the main flush periods.
Conclusion
Citrus leaf spot rewards preparation and punishes reaction, because every product available is protective and the main disease hides for months before it shows. Start with the orchard floor, since the leaf litter under the tree is where greasy spot is made. Prune for an open canopy, shift away from overhead irrigation, and put copper on the new flush before the wet season rather than after the blisters appear, aiming the spray upward so it wets the undersides of the leaves. And take a moment to name what you are looking at first: blisters on the underside are greasy spot, a yellow halo with dark corky veins is Alternaria brown spot, and raised corky lesions on both leaf surfaces are canker, which is bacterial and needs a different answer altogether.
Disclaimer: Always confirm current label registration, approved dosage, water volume, and pre-harvest interval (PHI) for your state before using any product, since labels are periodically revised by regulatory authorities.
This guide is a general advisory. Do your own research and verification before using any product. Always confirm suitability for your crop stage, region, and field conditions before making any purchase, spray, or crop-management decision.
