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Besides citrus canker and citrus greening, there are many other problems that affect citrus. Florida's humid climate harbors all kinds of fungi and creepy crawlers. The backyard citrus grower who is vigilant can spot these diseases easily and treat them early to prevent further damage. To be vigilant just walk around your yard whenever you think about it but at least once a week. Many of these problems are fungal diseases. Citrus canker and citrus greening are bacterial diseases.
Sometimes it is tiring to remember all the different symptom of the various fungal diseases. The most common fungal diseases of citrus in Florida are greasy spot, melanoses, citrus scab and foot rot. The first three affect fruit, leaves and twigs, the fourth, a disease of the "foot" of the tree near ground level directly affects the trunk of the three. Over time, foot rot can weaken the entire tree.
This article will address the first two fungal disease and leave the other two until a later article.
Imagine you injected drops of old, dirty engine oil below the surface of citrus leaves - you'd see "greasy spots" the kind the dry cleaner can't remove from your best white shirt after you changed a tire. These greasy spots are due to the cell tissue collapsing after the infection by the greasy spot fungus. With a moderate infestation, the leaves will function for 2 or 3 years after which there will be a gradual decrease in tree vigor. Weakened trees become more susceptible to other diseases and pests.
Once you have greasy spot, the cheapest, easiest, most effective control strategy is to remove and destroy fallen citrus leaves near the infected tree. Don't use fallen leaves as mulch, bury them, or put them in the horticulture pickup. Make sure they are picked up and destroyed before the summer rains. Oil and copper sprays can also control greasy spot. If you have a copper spraying schedule for citrus canker and citrus greening, greasy spot also will be treated. Copper sprays are the safest of the fungal sprays considering you are spraying a food crop.
Citrus scab is undoubtedly the ugliest disease of citrus appearing as a rash of scabs or warts on leaves, twigs and fruit. Since a new generation of spores can be produced within 5 days, citrus scab is a "compound interest" disease that can spread rapidly, given the right conditions, from overwintering infections on leaves and stems.
Compared to greasy spot fungus, the scab organism is an aggressive pathogen. Scab can first infect the citrus tree in early March and with the start of the summer rains march steadily all summer. Water from rain, overhead irrigation and dew is the most important factor affecting disease development.
Early stages of scab infection include well-defined conical growths on one side of the leaf – an identifying characteristic. These lesions may occur singly or be grouped irregularly. The crest of these warty growths usually become covered with a scabby tissue ranging in color from pale to dark. Badly infected leaves, twigs and young fruit can become misshapen with warty growths. The interior fruit quality is usually not affected.
The best scab management strategy is to remove and destroy infected leaves, twigs and fruit before the disease gains momentum. Put debris in the garbage and not the horticulture pickup. If possible, avoid overhead irrigation that spreads the scab spore by installing drip or micro-sprinkler irrigation. Copper sprays also can be applied 2-3 weeks after flower petal fall, and again 2-3 weeks later to control scab.
The Extension service has a free service of citrus home visits. It has been on "pandemic break" and hasn't been reinstated yet. You can call the extension service and ask for an appointment for a home visit.
Kathy Woodlock is Past President of the Garden Club of Cape Coral, a Master Gardner/Citrus Advisor and NGC Flower Show Judge.