Seasonal Leaf Drop
Being evergreens doesn’t make lemon trees immune to seasonal leaf drop. Unlike deciduous trees that lose their leaves in fall, however, lemons typically shed theirs in late winter or early spring. Why?
Because that’s when warming temperatures and increasing daylight hours where they grow in USDA plant hardiness zones 9 through 11 tell their new leaves to emerge! So, as long as you see that replacements are on the way for the leaves being lost, don’t worry.
Phytophthora Root Rot
A soil-borne disease present in most lemon groves, Phytophthora root rot slowly kills lemon trees by destroying their feeder roots. The fungus attacks the protective root cortex ’ so it pulls away from the inner tissue that transport water and nutrients to the upper tree.
As the infection takes hold, the leaves turn yellow and drop. The tree’s food supply is exhausted, its growth slows and its fruit production shrinks.
Managing Root Rot
To minimize the chances of root rot infection:
- Plant your lemon tree in very well draining soil.
- If that’s not possible, plant it on a 3- to 4-foot high raised bed.
- Water only when the upper 6 inches of soil are dry. Slow, deep watering with a soaker hose or drip system is best.
If your tree’s roots are severely damaged, remove and replace it with one on a Phytophthora-resistant rootstock. The best rootstock options are:
- ‘Cleopatra’ mandarin
- Sour orange
- Rough lemon
- ‘Carrizo’ citrange (a trifoliate orange hybrid)
Before replanting on the same site, treat the soil first with the organic Bacillus subtilis fungicide (strain QST713).
If your tree’s leaves are turning yellow or bronze and dropping at an alarming rate, suspect insufficient magnesium. Working Epsom salt into the soil corrects it. If the leaves turn yellow and drop while small, suspect iron deficiency and treat with iron chelates.
Yellow leaf mottling followed by heavy leaf drop are symptoms of citrus greening, a lethal disease transmitted by leaf-feeding Asian citrus psyllids. If they’re accompanied by small, lopsided fruit that remains green, citrus greening is almost certainly the problem.
Because it’s harboring the infection, you should remove, bag and burn the tree and all its roots.