Kumquat Pests and Problems: Scale, Mite, Leaf Drop and Sour Fruit

Scale insects, and the mold that follows them

Scale is the commonest serious pest on a potted kumquat, and the dense twiggy habit of the tree makes it easy to miss. Look for small brown, tan or white bumps, roughly an eighth of an inch (3 mm), clustered along stems, in leaf axils, and on the undersides of leaves against the midrib. They do not move and they look more like a blemish on the bark than an animal.

The first thing most people notice is not the scale but the stickiness. Scale excretes honeydew, which drips onto the leaves below and onto whatever the pot is standing on, and a black sooty mold then grows on the honeydew. The mold is harmless in itself but it blocks light from the leaf, and on a tree that is trying to ripen fruit through a dim autumn that matters. Ants running up and down the trunk are a reliable early tell, because they farm the scale for the honeydew and will actively defend it.

Wipe small numbers off with a cloth or a cotton bud dipped in alcohol. For anything more, spray thoroughly with horticultural oil, covering the undersides of leaves and the stem junctions, and repeat after two weeks to catch the crawler stage. Spray in the evening and never in strong sun or above 85F (29C), which will scorch the leaves. Deal with the ants at the same time or they will reintroduce the scale within a month.

Spider mite is a winter problem

Spider mite explodes in dry, warm, still air, which is precisely the condition of a heated room in winter. Since a kumquat is usually indoors or under cover from mid autumn onward with fruit still on it, this is the pest most likely to spoil a crop. The mites are too small to see clearly; what you see is a fine pale speckling across the leaf, which coarsens into a dull gray-bronze look, and eventually fine webbing at the shoot tips.

Prevention is mostly about the winter position. A cool bright room at 41F to 55F (5-13C) is close to mite-proof, while a warm living room at 68F (20C) is ideal for them. If you have no choice but a warm room, stand the pot on a tray of wet gravel, keep it away from a radiator or vent, and rinse the foliage in the shower every two or three weeks.

Treat with a soap or oil spray, thoroughly, on the leaf undersides, and repeat twice at weekly intervals; a single spray never clears them. Check monthly through winter by holding a sheet of white paper under a shoot and tapping it. Moving specks on the paper mean you have them, weeks before the leaves show it.

Leaf drop, yellowing and what they actually mean

Most people who think they have a kumquat pest have a root problem. The tree is shallow rooted in a small pot and it responds to almost everything the same way, by dropping leaves, so the pattern of drop is what tells you the cause rather than the drop itself.

Sudden drop of green leaves within days of a move is shock, from a change in light or temperature, and it corrects itself if you leave the tree alone in a stable position. Slow yellowing of old leaves from the bottom up, with wet compost, is overwatering and rotting roots; unpot it and check for dark, soft roots that smell sour. Yellowing between green veins is a nutrient issue rather than a water one. Crisp brown leaf edges with the compost dry mean it has been allowed to go too dry, probably repeatedly.

The one thing not to do is respond to leaf drop by watering and feeding more. That is the correct response to only one of the four causes above and actively worsens two of them. Establish first whether the compost is wet or dry, two inches (5 cm) down, and go from there.

Some leaf drop after moving a kumquat indoors for winter is normal and not a problem. Expect to lose perhaps a tenth of the leaves in the fortnight after the move. Losing half of them means the move was too abrupt or the room is too warm and dim.

Fruit that never gets sweet

The most common complaint about a home-grown kumquat is not a pest at all. It is fruit that colors up perfectly and still tastes harsh. Because the kumquat flowers late in summer, the fruit ripens through autumn and winter, and the sugar in the skin depends on light and warmth over exactly that period. A short, cool, dull autumn simply does not deliver it.

The fixes are all about extending the warm bright period. Keep the tree outdoors in the sunniest sheltered spot as late into autumn as your frosts allow, against a wall that holds heat if you have one. When it does come in, put it in the brightest cool position rather than the warmest one. And do not pick early out of impatience; the last few weeks on the branch contribute more sweetness than the first two months did.

Underfeeding contributes as well. A tree carrying several hundred fruit on a small root system runs short of potassium late in the season, and the result is pale, sharp fruit. A low nitrogen citrus feed through the second half of the growing season fixes this, stopping once nights turn cold.

Recording what happened and when

Pests run on a predictable annual cycle, and the record of when yours appeared is worth more than any general advice. Note the date you first saw scale, mite or aphid, and what you did about it. In most gardens the same pest turns up within a fortnight of the same point each year, which means the second year you can inspect and treat before there is anything to see.

Record the date you moved the tree under cover and how many leaves it dropped afterwards, roughly. Record the room and its temperature. When you are trying to decide next autumn whether the porch was better than the spare room, that note is the only evidence you will have.