For those with citrus, you are most likely anticipating a harvest of juicy goodness shortly. For others, you may be wondering why your citrus isn’t producing. Alternatively, perhaps you may be trying to decide if a citrus tree is for you. Here are a few of the more common questions and issues related to citrus.
Rootstock taking over
I often get asked “why am I getting very large lemons on one branch” or “this is not the variety I planted, why?” The answer is that the rootstock has taken over. Citrus is grafted – meaning the desirable cultivar “scion” is grafted onto a different rootstock. Grafting is done for two major reasons: disease resistance and quicker fruit production.
When the rootstock is not kept in check it will grow and outcompete the scion, sometimes to the extent that the scion will completely die, leaving only the rootstock. It is a good idea to do a regular rootstock check. If any growth is coming from below the graft line, cut it off. The graft is distinguishable as a slightly swollen area, however, sometimes it can be tricky to identify. The rootstock is almost always more vigorous than the scion and many times you will notice an extra tall or many cases extra thorny branch or two.
Yellowing of foliage
Yellowing is a symptom of numerous issues. Sometimes it is simply the plant moving nitrogen away from older leaves into the new spring growth. Most of the time the plant will balance itself out, but sometimes a bit of nitrogen or citrus fertilizer will help.
Iron deficiency results in green veins surrounded by yellow leaf, and is often caused by high pH. To ensure the pH is in a range allowing iron to be utilized, apply soil sulfur in spring and fall.
Overwatering will also cause yellowing, as nitrogen will be leached out of the soil. Citrus prefers to dry out a bit. If you are not sure if you are overwatering, always dig down several inches into the soil to check moisture.
Circling roots can cause yellowing and leaf drop on young trees. I often see this when citrus is planted in poor-draining soils. To check, take the base of the trunk in your hand and gently rock the plant back and forth. A plant in the ground for two years should not have much movement. If you notice movement under the soil (near the trunk), then there is a good chance the roots are circling, slowly killing the plant. In this case, dig up the plant, rearrange the roots, if possible, and replant in a new location with better drainage.
Certain diseases can cause yellowing of plants as well. For more information on these check out ipm.ucanr.edu.
Fruit isn’t forming
It can take several years for young citrus to produce a crop. For Meyer lemons this can be upwards of five years. Young plants often flower but the plant is generally not mature enough to handle a crop. Sometimes it seems as if fruit is forming, but in actuality these are the unfertilized ovaries falling off the tree. If a tree is mature, otherwise healthy and not forming fruit it could also be from: lack of pollinators, a strong drying wind during flowering or water stress.
Citrus leafminer tunnel into foliage
Leaf miners are the larvae of small moths which tunnel into the new foliage in spring (but can occur anytime during the year). As a result, they will leave squiggly lines and curling leaves on plants. Due to the tunneling aspect of the larvae, control can be difficult. In spring, when new growth occurs, spray the plant with soap spray, neem oil or even a garden hose every few days in hopes of knocking off eggs. That said, the best action is to allow natural predators to control. The good news is the pest generally causes very little damage, especially on a mature, well-established tree. For all trees (but especially young ones), avoid pruning the damage off as this will promote new growth.
Scale suck life out of plants
Citrus plants are susceptible to various pests, but a common nuisance are various species of scale. Most people are not aware they have an insect issue, but instead think the plant is oozing sap. Scale, as adults, are not mobile and look more like growths on the plant. Scale are sucking insects, meaning they pierce into the sugar transport part of the plant (phloem) and pull out photosynthates. In return, they excrete a sugary substance called honeydew. This honeydew is an ideal food source for ants, who will protect the scale from natural predators. To control scale, first control ants using a product such as Tanglefoot. Also, spray with a horticultural oil during the cooler season to eliminate any eggs or young.
Citrus greening disease
Recently introduced into California, it is important to be vigilant for citrus greening disease. This is a bacterium spread by the Asian citrus psyllid. Once infected, the plants will ultimately die within a few years. Quarantine is crucial. The Asian citrus psyllid is a small (aphid size) brown/gray insect. During the nymphal phase, long filamentous white cottony-like growth is noticeable. Citrus greening disease will cause ripe citrus to remain green, leaf mottling and branch dieback. If you suspect you have the Asian psyllid or citrus greening disease, you need to contact your local agricultural extension.