Raghuwinder “Raj” Singh

Citrus canker is a devastating disease of citrus caused by the bacterium Xanthomonas citri subsp. citri. It causes defoliation, premature fruit drop, blemished fruit and tree decline. It was first detected in Florida in 1910. By 1914, it spread to seven Gulf and Atlantic coastal states including Alabama, Georgia, Louisiana, Mississippi, South Carolina and Texas. In Florida, citrus canker was declared eradicated in 1933, but it reappeared in 1986 and then in 1995. It is now present in all citrus-growing counties in Florida. Until recently, the disease had not been seen in Louisiana since 1940; however, it was re-confirmed in the state on June 21, 2013.

Citrus is grown primarily in southeast Louisiana and includes grapefruit, kumquats, lemons, limes, oranges and satsumas. Commercial citrus production is centered in Lafourche, Plaquemines, St. Bernard and Terrebonne parishes. Citrus canker has been positively identified in Plaquemines Parish, where the majority of commercial citrus industry is located, along with Jefferson, Orleans and St. Charles parishes. More than 400 trees have been confirmed positive, and all of Orleans and portions of Jefferson, Plaquemines and St. Charles parishes are under quarantine. Consequently, the movement of citrus plants, plants parts, clippings or fruits is restricted. At present, the total value of citrus fruit and nursery stock industry in the state is around $10 million. Without effective management options, citrus canker has the potential to destroy Louisiana’s citrus industry.

Citrus canker is highly contagious, and all citrus varieties are susceptible – although some varieties are less susceptible than others. Varieties ranked from highly susceptible to less susceptible are grapefruit, trifoliate orange, Mexican/Key lime, navel orange, sour orange, sweet orange, lemon, satsuma, tangerine, Mandarin orange, king orange and kumquat. The bacterium causes symptoms on all above-ground plant parts, including leaves, fruit and twigs. Young expanding tissue is highly prone to infection, and as the tissue becomes mature and hardens off, it becomes less susceptible. Natural infection requires free water on the leaf surface to permit bacterial access through stomates or wounds. The pathogen prefers a temperature range from 68 to 86 degrees but is active over a wide temperature range. Lesions appear at about 10 days to two weeks after infection.

Symptoms on leaves and fruits start as tiny raised blisters that expand and become tan to brown as the disease develops. Lesions are visible on both sides of the leaves with water-soaked margins surrounded by yellow halo (Figures 1 and 2). As the lesion ages, the center becomes raised and corky and can fall out, giving the leaf a shothole appearance. The pathogen forms raised, corky, craterlike lesions on the fruits (Figures 3 and 4). Those fruit lesions often also have water-soaked margins surrounded by a yellow halo. Similar lesions are present on the twigs and leaf petioles, except the water-soaked margins may be reduced and the yellow halos are absent (Figures 5 and 6). As the disease intensifies, defoliation and twig dieback occur, and severely blemished fruit drop prematurely.

The bacterium enters the host tissue via natural openings and wounds. It is not vectored by insects or other Figure 2. Canker lesions on the underside of a leaf. Notice the young lesion with yellow halo and the older lesions with tan or brown margins. organisms, but the wounds caused by citrus leafminer may serve as infection sites. Bacteria survive in old cankers and under wet and warm environmental conditions. They can disperse short distances via wind-borne rain, lawnmowers, other landscaping equipment and people carrying the infection on their hands, clothing or equipment. Long distance dispersal of citrus canker generally is attributed to human movement of infected or exposed citrus material and storms like hurricanes and tornadoes.

Louisiana residents are urged not to move any infected citrus plant material within or out of the state. Homeowners must buy citrus trees from certified nurseries only. Individuals who believe their citrus trees have symptoms should not take any samples but contact the U. S. Department of Agriculture at 225-298-5410 or the Horticulture and Quarantine Division of the Louisiana Department of Agriculture and Forestry at 225-952-8100. More information about citrus canker can be obtained by contacting Raj Singh with the LSU AgCenter at 225-578- 4562 or rsingh@agcenter.lsu.edu.

Raghuwinder “Raj” Singh is an assistant professor in the Department of Plant Pathology and Crop Physiology.

(This article was published in the 2014 winter issue of Louisiana Agriculture magazine.)