Lemon leaf drop and citrus leafminer

Lemon tree is dropping its leaves, and an orange tree is infected with citrus leafminers

QWhy is my lemon tree losing its leaves, yet small fruit are still attached?

– Debra Korman, San Jose

AThere are a few reasons that lemon trees drop their leaves. Although we grow citrus in the Bay Area, we don’t have the best climate for it, and cold temperatures can cause what is known as winter leaf drop.

We’ve had some pretty chilly nights lately, so it might be your tree is responding to the colder temperatures. Experts recommend protecting the tree if you know the overnight temperatures are expected to drop into the freezing range — 32 degrees and lower.

Another common reason for leaf drop is overwatering. Check to see if your tree is sitting in wet soil or has an accumulation of water around the base. Too much water can cause root rot, which then leads to leaf drop. Check your irrigation system. Lemon trees are evergreen, but they need a little less water in the winter than they do in the summer.

A lack of nutrients can also cause leaf drop. The recommended application of a citrus fertilizer is once in January or February, again in April or May, and then in August or September.

If the leaf drop is small, it could be harmless. Lemon trees naturally drop a few leaves here and there throughout the year. Leaf drop, unless it is drastic, shouldn’t affect the lemons already on the tree, but it could mean they will be smaller, and you might not have as large a crop.

QMy mature orange tree — about 12 feet high — has been infected with citrus leafminer on all the new growth. I read that spraying with spinosad and mineral oil can help but it must be applied to both sides of the leaves. This does not seem practical given size of the tree and my small tank sprayer.

Do you have any thoughts on what I should be doing to control this problem?

– Tom Gleason, Bay Area

ABecause your tree is mature, it’s most likely to withstand any damage done by the leafminer. Very young trees are at the greater risk. Agricultural experts recommend doing nothing and letting natural predators of the leafminer do their work.

Insecticides, including the relatively safe spinosad, are not terribly effective in controlling the leafminer and can end up doing more harm than good by killing the insects that will prey on the leafminer. The predators lay their eggs inside or on top of the leafminer larva; when the eggs hatch, the parasite larva consume the leafminer larvae.

You can help reduce the infestation with cultural and physical practices. Citrus leafminer moths are attracted to the new growth on citrus, so don’t prune live branches more than once a year. The leafminers can’t penetrate older, hardened leaves. Although the damage you already have might be unsightly, avoid pruning it off as that will encourage new growth for the leafminers to attack.

You also should avoid applying nitrogen fertilizer in the summer and fall as that, too, will spur new growth. Once the leafminers have disappeared, you can go back to your regular pruning and fertilizing schedule.

One exception to the pruning rule is the removal of water sprouts — those vigorous and fast-growing branches that shoot up in mature trees. That’s often where the leafminer strikes first, so those should be removed when you see them, along with any suckers, which grow from the root stock and don’t produce any fruit that you want anyway.