By Richard F. Lee
The old-timers called citrus leprosis “nailhead rust.” Prevalent in Florida in the early 1900s, the disease was first called leprosis in the 1920s by H.S. Fawcett. Although the disease was widespread in Florida at one time, it mysteriously disappeared in the mid-1960s. L.C. Knorr [University of Florida/Institute of Food and Agricultural Sciences Citrus Research and Education Center (CREC)], in his annual research reports from the 1960s, last reported finding leprosis in 1965 in the Titusville area.
When I came to CREC in 1978, I made several field trips to the locations where Knorr had reported leprosis to occur, but could not find the disease. It has been speculated that leprosis, which is spread by Brevepalpus species mites, disappeared due to the freeze of 1963. At that time, growers often made sulfur applications; sulfur is an effective miticide.
This disease is now posing a threat to citrus in North America. With a recent find in Texas, Florida again faces the threat of leprosis. The disease has moved northward from South America through Central America, and has been reported in three citrus-production areas of Mexico. Leprosis devastated Venezuela’s citrus in the late 1990s, reducing it to a minor crop there.
The disease is named because of the brown, necrotic spots or lesions which occur on leaves, twigs and fruit. The spots on leaves are often chlorotic, with concentric rings and are present on the top and bottom of the leaf (Figure 1).
On symptomatic fruit, the spots are usually brown, reddish-brown or tan-colored (Figure 2). The lesions on fruit are limited to the fruit rind only and do not extend into the fruit sections.
The lesions on twigs and branches start as brown, concentric spots which enlarge over time, and larger lesions on branches can become scaly (Figure 3). Leprosis-affected trees produce small fruit and have premature fruit drop.
The viruses associated with leprosis disease are not systemic. Rather, the virus is localized at the margin of the lesion, and the lesion enlarges over time as the virus multiplies. Thus, a lesion on a twig will eventually girdle the twig, and the outer portion of the twig will die. Left untreated, leprosis disease will kill a mature citrus tree in about four years. Leprosis affects all citrus types (sweet orange, mandarin, lemon, grapefruit and citranges) as well as Swinglea glutinosa, a citrus relative.
Four viruses have been associated with leprosis disease thus far: citrus leprosis virus cytoplasmic type (CiLV-C), citrus leprosis virus nuclear type (CiLV-N) and two variants of CiLV-C, which are referred to as CiLV-C1 and CiLV-C2. The cytoplasmic type multiplies only in the cytoplasm of infected cells, whereas the nuclear type multiplies only in the nucleus of infected cells. The nuclear type and cytoplasmic type viruses can occur in the same plant. The symptoms caused by all four viruses are similar.
The disease is spread by Brevipalpus mites, commonly called broad mites. The Brevipalpus mites have a broad host range with over 900 plant hosts reported from 513 genera. Viruliferous mites have been shown to hitchhike on non-citrus hosts to new locations, then move over to citrus where they can still transmit leprosis. Movement of mites can occur by movement of fresh, untreated fruit, and mites can be windblown.
Mite control is essential for the control of leprosis. In Brazil, where leprosis has been endemic for many years, 12 or more miticide applications are made yearly. Leprosis-infected trees can be recovered with the pruning of the symptomatic branches, followed by good mite control, but the pruning of individual trees is expensive.
The symptoms of leprosis are distinctive; however, they could be confused with citrus canker lesions on leaves, fruit and twigs. Confirmation of the disease is by reverse transcription polymerase chain reaction (RT-PCR) assays for the four viruses associated with leprosis disease. Serological assays have also been developed, but as a rule, they are not as sensitive as the RT-PCR assays. Early detection of leprosis disease would provide the opportunity for eradication or at least suppression of the infection to keep it from becoming widespread in Florida.
Richard F. Lee was a professor of plant pathology at the University of Florida/Institute of Food and Agricultural Sciences Citrus Research and Education Center in Lake Alfred from 1978 to 2003. From 2003 to 2014, he was the supervisory research plant pathologist at the U.S. Department of Agriculture’s Agricultural Research Service National Clonal Germplasm Repository for Citrus and Dates in Riverside, California. He is now retired.
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