Fungi on plants: identification and treatment

Fungi on plants are the number-one cause of appearance problems in the garden: spotted leaves, white powder, shoots that rot, plants that yellow and defoliate early. And almost all fungal diseases share the same two triggers: prolonged moisture on the leaf and poor ventilation. A fungal spore needs several hours of liquid water on the leaf to germinate and infect; if that leaf dries quickly, nothing happens.

That's why prevention — how and when you water, how much you space the plants, when and how you prune, which varieties you choose — solves more than any fungicide. And natural fungicides (sulphur, copper, bicarbonate) are preventive: they create a barrier on the healthy leaf but they don't cure the already-diseased leaf or kill the fungus that is already in the tissue. This guide has a quick identification table; the treatment of the six most common diseases; how sulphur and copper are used and what their limits are; the key difference between a leaf fungus and a soil one; the seven prevention measures that really work; and a calendar of preventive treatments. At the end, twelve frequently asked questions.

Quick identification table

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Treatment by type

Powdery mildew

The easiest to control and the most common (rose, courgette, grapevine, sage, aster, lilac, strawberry tree). It's favoured by a contrast: dry hot days with cool humid nights. As soon as you see the white powder:

• Remove the most affected leaves and shoots and put them in the bin, not the compost.
• Spray wettable sulphur (never above 28-30 °C: it scorches the leaf) or potassium bicarbonate (a tablespoon per litre + a few drops of soap so it wets), every 7-10 days until it clears.
• Support remedy with some scientific evidence: diluted milk at 10-20% in water, sprayed on a sunny day (the milk proteins, in the light, generate antifungal compounds).
• Medium term: more air between plants, less nitrogen, watering at the base, and choosing resistant varieties when buying (there are courgettes, roses and grapevines selected for their tolerance).

Downy mildew

Much more aggressive and fast (tomato, potato, grapevine, lettuce, rose, spinach). It can devastate a tomato plant or a vineyard in a week of humid, mild weather. Act preventively:

• Apply copper (Bordeaux mixture, oxychloride) or potassium phosphonate as soon as the mild spring rains begin, before you see any symptom.
• Repeat after every rain (copper washes off) and every 10-14 days in the risk period.
• Once established it's almost impossible to stop: cut and destroy (bin, not compost) everything affected and protect the healthy part that remains. On tomato, remove the lower leaves so air can circulate.
• Water only by drip and in the morning; downy mildew needs a wet leaf to infect.

Copper and sulphur are contact fungicides and preventive: they create a protective film on the healthy leaf, but they don't "cure" the diseased leaf or kill the fungus that is already in the tissue. They have to be applied before infection or at the very first symptom, and renewed after every rain and as the plant grows and puts out new unprotected leaves.

Rust

Pustules of orange, brown or black powder on the underside (rose, mallow, geranium, garlic, leek, bean, plum). Remove and bin the leaves with pustules as soon as they appear, don't wet the foliage when watering, air it out with pruning and treat with sulphur or horsetail extract. On very prone plants, choose resistant varieties and don't plant them too close. With garlic and leek, rotate the crop and don't plant them where alliums grew in the two previous years.

Botrytis (grey mould)

A fuzzy grey mould that rots buds, flowers, soft fruit (strawberry, grape, tomato) and cuttings, entering almost always through a wound or a faded petal. It's a disease of hygiene and humidity:

• Remove faded flowers, damaged fruit and fallen leaves before they rot and become an outbreak point.
• Air it out: space the plants, prune to open the centre, ventilate the greenhouse in the morning to lower the humidity.
• Don't water from above or at dusk.
• Avoid pruning wounds in humid weather; if you have to prune, do it on a dry sunny day.

Rose black spot

The disease that most strips roses bare, especially in humid climates. The fungus overwinters in fallen leaves and in the bark. Plan a complete programme:

• Water at the base, never overhead.
• Meticulous collection of all fallen leaves (to the bin, not the compost) throughout the season.
• Winter pruning to open the centre of the bush and remove wood with spots.
• Preventive treatments with sulphur or copper from bud break and after every rain.
• Choose resistant varieties: today many modern and shrub roses are. The detail is in the rose guide.

Root and collar rots

The whole plant wilts with wet soil, the leaves droop and don't recover even if you water (in fact it gets worse when you water more). When you dig the plant up, the roots are brown, soft and smell bad, and the collar may be dark and sunken. It's almost always a problem of excess water and drainage, not a disease you "treat": the fungus (Phytophthora, Pythium, Fusarium) takes advantage of waterlogged, oxygen-starved soil. Solution: pull the plant up and don't replant the same species there, improve the drainage, plant on a raised mound and water much less. On valuable plants, you can try cutting the watering drastically and applying a Trichoderma-based product (antagonistic fungus).

Prevention: the 7 measures that really work

1. Water at the base and in the morning. Never wet the leaves at dusk. Drip irrigation is your best ally against fungi.
2. Space the plants. Circulating air dries the dew and rainwater before the spore has time to germinate (it needs several hours of wet leaf).
3. Prune to open the centre of shrubs and fruit trees. Light and air are free fungicides. Also remove the lower leaves of the tomato plant.
4. Clean up in autumn. Remove leaf litter, mummified fruit on the tree and debris where the fungus overwinters to reinfect in spring.
5. Don't overdo the nitrogen. The tender, watery tissue of fast growth is fungus food. Feed balanced and with potassium.
6. Improve the drainage if you have rots: the real problem is almost never "the fungus", it's stagnant water and lack of oxygen at the root.
7. Rotate the veg crops (don't repeat botanical family for 3-4 years) and disinfect the secateurs with alcohol when going from a plant with symptoms to a healthy one.

Calendar of preventive treatments

• Late winter: copper treatment on stone and pome fruit (against shot hole, peach leaf curl, scab), and on olive (peacock spot).
• Bud break (March-April): first sulphur on roses prone to black spot and powdery mildew. Preventive copper on grapevine at bud break.
• Humid spring: copper on tomato and potato as soon as the rains begin, repeating after each one and every 10-14 days.
• Summer: sulphur against powdery mildew on courgette, grapevine and rose, always below 30 °C and at dusk.
• Autumn: copper on fruit trees after leaf fall to reduce the overwintering inoculum. Thorough collection of rose leaves.

The "infection risk": the wet-leaf-hours rule

Most leaf fungi need the surface to be continuously wet for a minimum number of hours and at a specific temperature for the spore to germinate and penetrate. It's the basis of the plant-health warnings that some regions publish for vineyards or olive groves.

The practical lesson: if you can keep the leaves never wet for more than 4-6 hours (watering by drip at dawn, spacing the plants and pruning to air them), most of these fungi can't even get started, and you save yourself almost all the treatments.

How to take and send a sample for diagnosis

If a problem keeps coming back and you can't identify it, many cooperatives, nurseries and regional plant-health services offer diagnosis. For it to be any use:

1. Take several leaves or twigs with the symptom at different stages (starting, medium, advanced), not just the worst ones (sometimes they're already colonised by secondary fungi that throw you off).
2. Wrap them in slightly damp newspaper, not in sealed plastic (they rot).
3. Note down: species and variety, age of the plant, when it started, whether it has rained or you have watered overhead, what you have applied and with what result, and a photo of the upper and lower surface in good light.
4. For a suspected soil fungus, also include a piece of root and collar, and some of the surrounding soil.

Frequently asked questions

Can I use copper and sulphur in an organic veg garden?

Yes, both are approved in organic farming, but with limits. Copper accumulates in the soil and is toxic to soil life (worms, microorganisms) in excess: no more than 2-4 applications a year and respecting the dose. Sulphur is more benign but scorches the leaf in heat. Always respect the safety interval before harvesting.

The mushrooms that come up in the lawn or the mulch, are they a problem?

No. They're saprophytic fungi that break down dead organic matter and they even indicate a living, healthy soil. Remove them if they bother you aesthetically or if there are small children (some are toxic if eaten), but they don't harm the plants. "Fairy rings" (rings of mushrooms or greener grass) in the lawn are controlled by spiking and thoroughly watering the area with a little soap, though they're hard to eradicate.

Does bicarbonate work against powdery mildew?

Potassium bicarbonate works well and is kinder to the plant than sodium bicarbonate, which in repeated doses salinises the soil. Dilute a tablespoon per litre of water and add a few drops of soap so it wets the leaf. Apply preventively and at the first symptoms, every 7-10 days.

How do I know if it's a leaf fungus or a soil one?

Leaf fungus: spots, powder or pustules localised on leaves and shoots, the plant stays alive and gradually loses leaves. Soil fungus: the plant wilts entirely all at once with wet soil, doesn't recover when watered, and when you cut the base of the stem the internal channels are dark. Soil ones have almost no cure: you have to pull the plant up and not replant the same thing there.

Can I compost diseased leaves?

Better not, unless your compost reliably reaches high temperatures (55-65 °C), which destroy most spores. In a normal home composter, which doesn't always heat up enough throughout the mass, leaves with black spot, rust or downy mildew go in the bin so as not to reinfect the garden when you use the compost.

How often do I need to repeat a sulphur or copper treatment?

Every 7-14 days in the risk period, and always after rain, because they wash off. They're barrier products: if the barrier goes with the water or the plant puts out new uncovered leaves, it has to be replaced.

Do vinegar or hydrogen peroxide work as a fungicide?

Vinegar doesn't; it can scorch the leaf and its antifungal effect is minimal and very short. Very dilute hydrogen peroxide is sometimes used against soil fungi in seed trays, but with little solid basis. For leaf fungi, sulphur, copper and potassium bicarbonate are what work.

Can I prevent fungi without using any product?

To a large extent, yes, with the 7 prevention measures: watering at the base in the morning, spacing, pruning to air, autumn clean-up, not overdoing the nitrogen, good drainage and resistant varieties. Many well-planned gardens hardly ever need to treat.

Does wood ash work against fungi?

Dusted onto the damp leaf, it forms a physical barrier that slightly hinders powdery mildew and snails, but it's a minor remedy and in excess it makes the soil alkaline. It doesn't replace sulphur.

And if I buy a plant and it has fungi?

Isolate it from the rest (quarantine), remove the affected leaves, treat according to the fungus and don't plant it near other susceptible plants until it's clean. Always check plants at the nursery before buying them: leaves top and bottom, and the state of the collar.

And if I'm not sure it's a fungus?

If you see insects, sticky honeydew or stippling with webbing, it's a pest: see common pests and natural solutions. If the leaves yellow uniformly with no spots or bugs, it's usually watering or soil pH and nutrients. When in doubt, take a detailed photo of the upper and lower surface and get it checked; the right diagnosis saves useless treatments and money.