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Check citrus trees now for signs of distress, disease or pests

Check citrus trees in summer for signs of distress, disease or pests.
Check citrus trees in summer for signs of distress, disease or pests. (San Luis Obispo) Tribune file

It can be a bit alarming to look out at your lemon tree in June and see the ground underneath littered with tiny fruit and dry yellow leaves. “June drop” is a normal process for citrus trees. Citrus trees set many more fruit than the tree can hold and in June the trees shed the excess fruit and leaves.

Do take a look at the remaining leaves to check for signs of nutrient deficiencies. Yellow leaves with green veining indicate an iron deficiency. A nitrogen deficiency shows as light green new leaves and yellowing older leaves that eventually fall off. Yellow leaves that show wide green margins along the veins can indicate deficiencies in manganese – a fairly common problem in poorly drained or heavy clay soils with high pH levels as we have in the central San Joaquin Valley.

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Good quality citrus foods contain high percentages of essential nutrients and trace minerals. Look for a formulation that also contains sulphur which lowers soil pH levels, allowing roots to be better able to take up iron. Sulphur in the form of granules can be applied separately if the citrus food lacks sulphur. Follow recommended application rates and irrigate well before and after applying any fertilizer. Within two to three weeks after application, nutrient deficiencies should be corrected and leaves should revert to the normal dark green. Citrus foods with sulphur will also correct nutrient deficiencies in other trees and bushes including gardenias that often are lacking in iron here in the Valley.

While you’re checking leaves for nutrient deficiencies, poke your head inside the canopy and look for strong green shoots growing straight up through the canopy and for suckers along the trunk. Suckers and shoots deplete the tree’s energy and should be removed. Wear gloves; the suckers often have sharp spines. Many citrus will have a lot of dead twigs and small branches inside the canopy. Remove these, as well, to improve air circulation, clear hiding spots for pest insects, and to provide access to the canopy interior for beneficial insects. Do not remove branches that form the outer canopy; they hold the fruit and shade the trunk.

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Citrus scale are hard to see. Look for small immobile brown bumps on twigs, along leaf stems and on branches. Mature citrus scale have a hard outer shell which is impermeable to insecticides. Apply a second spray of a lightweight horticultural oil now (the first application should be in May) to smother the soft-bodied immature or “crawler” stage of scale. Apply horticultural oils only in the early morning, late evening or night to protect bees. Read product labels for cautions on spraying when temperatures are high.

The scale suck out plant juices and excrete “honeydew” which attract ants. If you see ants in your citrus tree, use baits or barrier traps to control them; the ants actually herd the scale and drive off predatory beneficial insects in order to feed on the honeydew.

The leaves of citrus trees infected with Huanglongbing or HLB will show uneven yellow and green blotching. HLB is a bacterial disease carried by the Asian citrus psyllid. Check the UC IPM website, searching for HLB or Asian citrus psyllid, for good photos and complete information on identifying the symptoms of the disease and the psyllid that carries the fatal citrus disease. And if you suspect that your tree might be infected with HLB, contact your county agricultural commissioner’s office.

Elinor Teague: etgrow@comcast.net
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