The disease citrus greening, sometimes referred to as Huanglongbing or HLB is believed to be caused by the bacterium Candidatus Liberibacter asiaticus. Huanglongbing is Mandarin for “yellow dragon disease” which describes the yellow new shoots sometimes seen in infected trees.
This incurable disease has been in the news a lot over the past several years since it was first discovered in the Homestead area of Florida in 2005. HLB can be spread by an insect called the Asian citrus psyllid (ACP) or by humans through budding and grafting infected material. The ACP has piercing sucking mouthparts that allow it to acquire and transmit the greening pathogen from the vascular tissue of infected HLB hosts to healthy host trees. HLB host plants include all members of the citrus family including ornamental relatives the box orange and orange jasmine. The adult Asian citrus psyllid is widely distributed in tropical and subtropical Asia. Over recent years citrus greening has become a worldwide problem, significantly affecting citrus production in several countries including Asia, Africa, the Arabian Peninsula, and Brazil. Citrus producing areas of the world that remain unaffected by greening are Australia and the Mediterranean Basin.
Progression of citrus greening in Florida
To fully understand the history of progression of citrus greening in Florida we need to go back to 1998 when the ACP was first discovered here. It is believed that ACP arrived in Florida before the citrus greening pathogen did. From the time ACP was first discovered in Florida, it only took three years for the insect to find its way to 31 Florida counties. By the time greening was detected in south Florida seven years had passed. The perfect storm scenario had been created due to the extensive spread and establishment of the ACP throughout Florida. We have all the pieces in place for a tough fight against greening, we have the insect that spreads the disease, we have a favorable environment for the insect to reproduce, and we have plenty of greening infected host trees.
Scouting for psyllids and greening in your citrus
To scout for psyllids and greening in your citrus trees look for misshapen new leaves and the immature stage of the insect called nymphs along tender new shoots. The wingless flattened nymphs of ACP range in color from yellow orange to brownish. You will need a magnifying glass to see them well. The presence of immature nymphs can be a good indicator that your trees are being subjected to feeding and greening transmission. By the time you notice a tree is sick it is likely other trees in the area have already been exposed to feeding from infected psyllids. Early stages of greening could be confused with a general lack of vigor or nutritional deficiencies from not enough or the wrong type of fertilizer. Greening infected leaves have a mottled appearance that is nonsymmetrical, in other words if the leaf is folded in half one side is not the mirror image of the other. This is in contrast to nutritional deficiencies that are usually restricted to the interveinal areas of the leaf and match up when the leaf is folded in half along the midvein. Trees become stunted, suffer twig dieback, and produce off season blooms. During the final stages of the disease the fruit becomes misshapen or may remain small and green and taste bitter. When citrus trees quit serving the intended purpose of providing edible fruit or pleasing aesthetics they should be removed. For more information please visit http://edis.ifas.ufl.edu/pdffiles/PP/PP32600.pdf “Frequently Asked Questions About Huanglongbing (HLB; citrus greening) for Homeowners”, http://edis.ifas.ufl.edu/pdffiles/PP/PP32800.pdf “Huanglongbing (HLB; citrus greening) and Nutrient Deficiency Identification”, and For more information please visit http://edis.ifas.ufl.edu/pdffiles/IN/IN16000.pdf “Asian Citrus Psyllid, Diaphorina citri Kuwayama (Insecta: Hemiptera: Psyllidae)”
What homeowners can do to help citrus continue to produce well is pamper their trees by providing optimal growing conditions and supplying a well-balanced citrus fertilizer.
Citrus trees require well drained soil. This means proper site selection for planting and irrigations scheduling to account for rainfall can help prevent root rot pathogens. Homeowners should also consider using horticultural oils to treat to prevent insect infestations on their citrus trees. Horticultural oils work by suffocating insects by clogging their breathing spiracles. It is important to coat both sides of the leaves with oils for effective insect control. Horticultural oils can also prevent insects from laying eggs because their residue leaves leaf surfaces slippery. Horticultural oils are very safe, but caution should be exercised to avoid spraying oils during bright sunny days due to a risk of burning tender leaf tissue. Always follow the label instruction on any product very carefully. For more information please visit http://edis.ifas.ufl.edu/pdffiles/PP/PP33600.pdf “Citrus Tree Care for the Home Gardener in the HLB Era”
Not sure if you have citrus greening or a nutrient deficiency?
Check out this website for some helpful images.
Sol Looker is the Residential Horticulture Extension Agent and Master Gardener Coordinator at the Flagler County Extension Service, University of Florida. For more information email him at email@example.com