What to do this week –By Mark Govan, Host “Florida Gardening”
Over the last several weeks I have been inundated with questions regarding citrus trees, their diseases, and the proper care they need to produce a good crop. In order for me to give you the information you need to care for your trees, you must first be able to distinguish disease problems and disorders from the insect problems which effect citrus trees. In this article I will share with you some of the information I have acquired over the years in the identification and the control of these problems. Because there is a lot of information to go over I will divide this information up into several articles that you can save and use as you look at your trees. When you see a problem on your trees, use this information to properly identify the cause of the problems and treat them accordingly.
Many of the diseases and disorders I am going to go over need to be treated at specific times of the year so you may want to have a calendar in front of you to mark specific times you will need to treat these problems. This week we will go over diseases and disorders of citrus. My next article will include information on how to identify specific insect problems and their control.
Algal Leaf Spot or Cephaleuros virescens is a summer and winter disease which appears during periods of heavy rain or dew. When you inspect your trees you need to look for raised white spots or patches on the leaves, twigs, or branches. These white blotches will sometimes coalesce and can form large patches which look similar to a cottony cushion scale insect problem. As the colony of algal spot ages, the white puffy patches can take on a brick red color. This disease can and will lead to limited shoot growth and dieback of the tree if left unchecked. This problem and a few others I will mention later can be easily controlled with applications of a citrus spray copper applied to the branches, stem, and leaves every two months. Make sure you follow the label directions. Mix the copper with water and spray the tree staring with the trunk, then continuing up the tree being sure to cover the twigs and leaves. Spraying should be done in the morning following a thorough watering of the tree the night before.
Another problem I see rather frequently on citrus trees is called Greasy Spot or Mycosphaerella citri. Symptoms of Greasy Spot are yellow spots on the upper leaf surfaces. Some people may think they have Citrus Greening, another disease I will go over later, but greasy spot can easily be identified by the small dark circular spots on the underside of the leaves. These spots do not come of if you rub your finger over them and have a greasy looking appearance. This disease normally shows up in the summer months but most of us do not notice the problem until the fall when the leaves of the tree begin to fall in mass. Orangesand limes are very susceptible to this problem. Although this disease will normally not kill the tree, severe defoliation can cause the tree to become stressed out allowing other insects or diseases to enter the tree. Control of this disease is the same as for Algal Leaf Spot, spray with a citrus copper you can find at most garden centers. I have had good luck timing my applications to when my tree puts out new flushes of growth or every couple months if you want to mark your calendar.
As I mentioned above, there is another problem which has been a cause of alarm for anyone growing citrus in the Tampaarea where this disease has become rampant. This problem is called Citrus Greening or Huanglongbing. Citrus greening is thought to be caused by a bacterium carried by an insect called the Asian citrus psyllid. Once a tree is infected by this disease, there is no cure but the slow decline of the tree may take several years.
Citrus greening symptoms start slowly eventually rendering the tree unproductive. All varieties of citrus can be affected. Early signs of citrus greening include “Yellow Leaf” which is new shoots growing from the top of the plant are yellow in appearance but this may not happen on all tress. In Florida, the leaf symptoms include a yellow mottled appearance which is not symmetrical. This means that if you fold the leaf at the midrib; both sides will have yellow blotches which do not match up with each other. This variegated chlorosis of the leaf can be confused with insect damage but when examining both sides of the leaf there will be no other insects or disease on either the top or bottom of the leaf. Over time, this mottling of the leaves will spread throughout the tree with some twig dieback also occurring.
As the citrus tree begins to decline, the fruit symptoms include misshapen or lopsided fruit. If you cut into a fruit, one side of the orange will be larger than the other. You may also notice smaller or fewer fruit on the tree. Those trees severely affected by citrus greening may also produce fruit which may turn yellow where it is attached to the tree with the rest of the fruit remaining green. The best time to scout for this problem is in the early spring and fall as symptoms can be hard to find during the active growing months. Plantings of Orange Jasmine or Orange Boxwoods, which are also hosts to the psyllid which transmit this bacterium, should be discouraged around citrus.
Melanose or Diaporthe citri is another disease of citrus trees which can be controlled with routine spraying of copper. Melanose appears as reddish-brown to black bumps on the upper leaves of citrus. These raised bumps are easy to feel as you rub your hand or thumb across the leaf surface. The bumps will have a sandpaper feel to them and they do not rub off as you glide your fingers across them. Twigs, stems, and fruit can also be affected in severe infestations. If left uncontrolled, extensive areas of scar tissue on the fruit may develop. This can also cause the fruit to crack as the fruit enlarges and eventually drop from the tree. The best time to control this problem with copper is right after petal fall in the spring. Make sure most of the blossoms have fallen off and use a low pressure spray to assure you do not knock off any developing fruit. As always, spray in the early morning after watering your tree the night before.
One of the easiest diseases of citrus to identify is called Scab or Elsinoe fawcetti. Scab looks like wart like growths or scars on the fruit or leaves of Temple, Murcott, Lemons, Tangelos, and Grapefruit trees. Leaves look to be twisted in appearance sometimes even distorted or misshapen. The fruit will have corky projections which form scabs over the surface of the rind. Pruning of the diseased parts of the tree and destroying followed by spraying with copper is the control. If you have had this disease in the past, routine sprayings with copper every two months will help limit this disease. I monitor my trees and spray them at the first flush of new growth following two thirds petal drop in the spring and I have had only limited problems.
Many of the diseases I have listed above require the same control, routine applications of copper at each flush of new growth or after petal drop in the spring. If you mark your calendar to remind yourself to spray, then you can eliminate most of these diseases fairly easily.
I hope you are not discouraged by the problems I mention above. Many of these problems have been around for years and are easy to control with routine inspections and spraying. Although there is no control for the citrus greening, with regular maintenance even those trees may last another seven years. In my next article I will go over some of the insect problems of citrus and their control. Thank you for your support and remember, without plants we would not be here!