Gadenin日本語Get the appLook at the new growth, not the fruit
Almost everything that troubles this tree starts on the soft new flush. Aphids, leaf miner, scale crawlers and mite damage all appear on shoots that are still extending, and by the time you notice a problem on the fruit the population has been building for a month. Get into the habit of turning over a few young leaves at the shoot tips whenever you walk past.
Kabosu makes a big tree with more new growth than a small sour citrus and it flushes two or three times a season, so there are more of these windows and each one needs the same quick check. The flush right after a heavy feed is the one most likely to be colonised.
| Problem | First sign | Where to look | What to do |
|---|---|---|---|
| Aphids | Curled, distorted shoot tips | Undersides of the newest leaves | Wash off with water; cut out badly curled tips |
| Citrus leaf miner | Silvery winding tunnels, twisted leaves | New flush leaves only | Leave the leaves on; remove nothing but the worst |
| Scale insects | Sticky leaves, then black sooty mold | Undersides of leaves and along stems | Scrape off, treat with horticultural oil out of hot sun |
| Spider mites | Fine speckling, dull leaves, webbing | Leaf undersides in hot dry spells | Raise humidity, hose the canopy, improve watering |
| Root rot | Whole tree pale, leaves drop, no new growth | Soil at the base after rain | Improve drainage; stop watering; it may not be savable |
| Canker-type skin lesions | Raised corky spots on fruit and leaves | Wind-damaged and thorn-scratched fruit | Remove affected fruit; improve shelter; disinfect tools |
The thorns are a disease problem, not just a safety one
This is specific to a heavily thorned citrus and it is the thing most guides miss. On a windy day the branches move, and the thorns on one branch scratch the fruit on the next. Every scratch is an entry point for the bacteria that cause the raised corky lesions people call canker, and the marks show up on the fruit skin weeks later when it is too late.
Two things fix it. Thin the canopy hard enough that branches are not lying against each other, and give the tree real shelter from the prevailing wind. On a young tree in an exposed site a windbreak on the windward side for the first few years is worth more than any spray.
Scratched fruit is still perfectly usable for juice; it just will not store well. Sort it out at picking and use it first.
Fruit drop is usually not a pest
Most people who report a problem with kabosu are describing fruit drop, and most fruit drop is physiological. The tree sets far more fruit than it can fill and sheds the surplus a few weeks after flowering. That drop is normal and healthy, and trying to prevent it does harm.
The drop that is a problem comes later, in mid summer, and it has three usual causes: the soil dried out and then got soaked, the tree is carrying more fruit than its leaf area can support, or a heavy nitrogen feed pushed the tree into growing shoots at the expense of the crop. Even watering, a sensible thinning after the natural drop, and no strong feed during the swell prevent nearly all of it.
If fruit is dropping with a brown ring at the stalk end and a soft patch, that is different: check for a rot getting in at the button, usually on fruit that was scratched or that sat in a wet pocket of dense canopy.
Cold damage and how it presents
Kabosu takes a light frost on hardened wood without much trouble but is not a cold-hardy citrus. Damage shows first as leaf edges going brown and papery, then whole leaves dropping, then bark splitting on the youngest shoots. The tree very often looks worse a fortnight after the cold event than it did the next morning.
Do not cut anything until the new growth starts. Wood that looks dead in cold weather frequently shoots again from the base, and a cut made in the cold invites dieback further down than the frost reached. When growth restarts, scratch the bark of each doubtful shoot with a thumbnail: green underneath means live, brown means cut back to green.
The tree most likely to be damaged is the one that was fed nitrogen late in the season and went into the cold carrying a flush of soft growth. Stopping nitrogen after mid summer is the single most effective piece of cold protection there is.
Keep a short log of what you saw and when: the date aphids first appeared, the date the natural fruit drop finished, the date of any cold damage. Pests arrive at broadly the same point in the season each year, and a two-year record turns a reaction into a scheduled check.