Most leaf disease on indoor and potted plants comes from a handful of fungi: powdery mildew, downy mildew, rust, leaf spot disease and blackspot. Each one looks different, and all of them need the same two conditions, a leaf that stays wet and air that does not move. Both are easy to create indoors. Support placed at the roots works from inside the plant, with nothing sprayed into the room.
How does something placed at the roots help a problem on the leaves?
It is placed in the soil near the roots rather than onto the leaf, and its ingredients release gradually into the root zone. There they support the plant's own restorative capacity, its vitality and its resilience.
The logic is the plant rather than the fungus. A strong, well balanced plant copes better with external pressure and is less susceptible to leaf disease pressure in the first place. That is a different idea from a coating that sits on the leaf surface, and it is also why nothing has to be sprayed into a room where people sit.
Why will the spots on my leaves not go away?
Because that tissue is already damaged, and nothing brings it back.
This is the single most common reason people think a treatment has failed. The marks you were looking at on the day you started will still be there in three weeks. Look at the leaves that have opened since. Clean new growth is the signal. Fresh marks on new growth are the signal to look again.
What is the difference between powdery mildew and downy mildew?
Powdery mildew is a white, powder like coating on leaves and sometimes stems. It wipes off with a finger. It develops in warm conditions, which describes most centrally heated rooms, and affected leaves can end up distorted, discoloured or dry.
Downy mildew develops inside the leaf tissue, which is what makes it different. You see yellow or discoloured areas on the upper surface, with fungal growth underneath the leaf that does not wipe away. Warm, humid conditions encourage it and help it spread.
The practical difference: if it wipes off, it is sitting on the surface. If it does not, part of it is inside the leaf.
Is the orange on my leaves rust?
Probably, if it shows as yellow, orange, brown or reddish spots or small raised pustules on the leaves. Rust is particularly common in humid conditions and can weaken a plant once the infection is severe.
A useful check: rust rubs off slightly as a fine coloured powder. A permanent stain or a patch of discolouration does not.
Why does my plant keep getting the same brown spots?
That pattern usually points to leaf spot disease, which is not one organism but several different fungi producing brown, black or discoloured spots. When it gets severe the affected leaves yellow and drop early.
It is the stubborn one because it keeps returning to the same plant if the source is never cleared. Taking the affected leaves off, rather than leaving them on the plant or lying on the soil surface, reduces how much spore material is sitting there waiting for the next flush of growth.
What causes small black spots that make the leaves fall?
Dark brown to black spots followed by early leaf drop is typical of blackspot. It is seen most on roses, including roses grown in containers, and on other ornamentals kept in consistently damp conditions. In a severe case the leaves yellow and fall, and the plant is weakened by losing them.
What about Alternaria and shot hole disease?
Both are worth recognising even though they turn up less often indoors.
Alternaria is a group of fungi causing dark spots and lesions on leaves. It affects tomatoes and potatoes as well as a range of ornamental plants, so it is one to know if you grow edibles in a greenhouse or on a balcony.
Shot hole disease is associated with the Prunus family: cherries, ornamental cherries, peaches, plums and apricots. Small spots appear first, then the affected tissue dies and falls away, leaving the holes that give it its name.
Why do indoor plants get leaf disease when they never see rain?
Because rain was never the requirement. A wet leaf or still, humid air is enough, and indoor growing produces both without anyone noticing.
Three everyday habits recreate exactly what these fungi need. Watering from above, so droplets sit on the leaves for hours. Water collecting in the crown of the plant. And pots pushed together on a windowsill or shelf with no air moving between them.
Watering at the base instead, and leaving a little space between plants, removes both triggers and costs nothing.
How do I treat leaf disease on a potted plant?
Make a hole about 5 centimetres deep in the soil near the roots. Put the capsule in, add a little water, and close the hole properly with potting soil.
Use 1 capsule per 2.5 litres of pot volume. A 2.5 litre pot takes 1 capsule, a 5 litre pot 2, a 10 litre pot 4, a 20 litre pot 6 and a 30 litre pot 8. Repeat after about four weeks if it is still needed.
How many capsules does a plant in open ground need?
That goes by the height of the plant rather than anything else. Make the hole about 10 centimetres deep near the root zone, put the capsules in with a little water, and close it over with soil.
A plant around 25 centimetres tall takes 1 capsule. 50 centimetres takes 2, 100 takes 4, 150 takes 6, 200 takes 8, 250 takes 10, 300 takes 14, 350 takes 18, 400 takes 20 and 450 takes 24.
For a larger plant, spread the capsules around the root zone rather than putting them all in one hole, roughly 25 centimetres apart where the space allows. Repeat after about four weeks if needed.
Should I use capsules or a spray?
It depends on where the plant is and how many of them there are, and the answer is simpler than it looks.
Indoors, capsules every time. Nothing goes into the air in a room you are sitting in, and there is no overspray onto a shelf, a windowsill or a sofa.
Potted plants outdoors, capsules as well. A pot holds a contained amount of soil, so a capsule placed in it reaches exactly the roots you are aiming at and nothing else.
One plant or a few plants in open ground, capsules again. Place them by the height of the plant and spread them around the root zone.
A whole border, a lawn, or a plot with a lot of plants on it, a concentrate you dilute and spray or pour over the area. Capsules are dosed per plant and per pot, with no per square metre method, so covering ground with them is the wrong shape of product for the job.
Is it safe around children, pets and vegetables?
The capsule is for use on plants only. Keep the pack out of reach of children, do not exceed the stated dosage, and treat it as what it is, a garden product rather than anything to be eaten by a person or an animal.
It is placed in the soil rather than sprayed over foliage, and the listed uses include vegetable gardens, greenhouses, terraces and balconies. Follow the instructions on the pack and apply it at the root, not to the part of the plant you are going to eat.
What is actually in a capsule?
Plant based oil with a concentrated botanical extract of Allium sativum at 100 to 1, equivalent to 1,500 milligrams of plant material per capsule. Allium sativum is garlic.
That concentration is the reason a small capsule does a month of work. It also means the contents smell of what they are made from, which is one more reason to close the hole over properly with soil rather than leaving it open.
How often should I treat, and what if I miss one?
The effect lasts about four weeks. Place the next capsule as the first one is ending and you have covered eight weeks without a gap, which is usually enough for one full stretch of pressure.
Missing a round does not undo anything the first capsule did. Support simply lapses until the next one goes in, and whatever is around has an easier time in the gap.
What if a treated plant still is not improving?
Check the new growth specifically. Old marked leaves never recover, so judging by them will always look like failure. If the leaves opening now are clean, it is doing its job even while the plant still looks patchy.
If new growth is coming through marked as well, something else is going on. A photo is the fastest way to find out what.
How do I stop leaf disease coming back?
Water at the base rather than over the leaves, and do not let water sit in the crown of the plant. Give plants enough space that air can move between them. Take badly affected leaves off and put them in the household waste rather than the compost, so the spores do not travel to a healthy plant nearby. Healthy leaves and prunings from unaffected plants compost as normal.
Those two conditions, a wet leaf and still air, are what every one of these fungi needs in order to establish. Remove them and you have done the most effective thing available, for nothing.
Still not sure what you are looking at?
Send a photo to support@organifer.com. Alex will tell you honestly what he sees and what he would do, including when the answer is that nothing needs treating. It often is.
Written by Alex, plant health specialist on Organifer's advice team.
