Citrus Diseases Exotic to Florida: Mal Secco1
K.-R. Chung2
Introduction
Citrus is susceptible to a large number of diseases caused by plant pathogens. Plant diseases that affect citrus can cause severe economic losses, but, fortunately, not all pathogens attacking citrus worldwide are present in Florida. Any exotic disease, if introduced, has the potential to significantly increase production costs and thus decrease profitability for Florida growers. This series of EDIS fact sheets (available at http://edis.ifas.ufl.edu/topic_series_citrus_diseases_exotic_to_florida) presents background information for each exotic citrus disease in order to provide a basis for evaluating exotic pathogens that may pose potential risks to Florida citrus and to facilitate a decision-making framework for preventing their introduction and spread. This paper discusses mal secco (Italian for dry disease), which is caused by the fungus Phoma tracheiphila (synonym: Deuterophoma tracheiphila).
Concerns about Mal Secco
Mal secco is a fungal disease that causes serious damage to a number of citrus cultivars in the Mediterranean and Black Sea areas (with the exceptions of Spain, Portugal, and Morocco). The fungus spreads systemically in the tree after initial infection and then kills infected limbs. It often causes a complete collapse of the whole tree if infection occurs on the trunk or low in the canopy. The disease has been known for a century in the Mediterranean area, but has not been reported in other citrus-growing regions.
A sour orange leaf affected by mal secco disease, showing vein chlorosis. Credits: Reprinted with permission from Figure 2 of Q. Migheli, et al., “Mal Secco Disease Caused by Phoma tracheiphila: A Potential Threat to Lemon Production Worldwide,” Plant Disease (2009) 93: 852-67. Courtesy of American Phytopathological Society (APS) Press.
[Click thumbnail to enlarge.]
Causal Agent of Mal Secco
The causal agent of mal secco is the fungus Phoma tracheiphila (syn. Deuterophoma tracheiphila). P. tracheiphila can be easily isolated and cultured from infected citrus trees. Isolates vary in pathogenicity and pigment production. Some less virulent types of P. tracheiphila have also been reported.
Cultivars Affected by Mal Secco
Under favorable conditions, mal secco can affect a range of citrus cultivars and relatives, including Fortunella, Poncirus, and Severina. However, mal secco is most severe on lemon, citron (Citrus medica L.), lime (C. latifolia Tan.), and bergamot (C. bergamia Risso). Under greenhouse conditions, P. tracheiphila is able to infect almost all citrus species that are wounded before inoculation. In fields, the fungus rarely affects oranges and grapefruit. However, P. tracheiphila infections of some tangelos, tangors, and mandarins have been reported. While infections most commonly occur in the scion, infection can also occur in rootstock sprouts. Rough lemon, alemow, sour oranges, volkamer lemon, and citranges are all susceptible. Trees of any age can become infected, but the disease is especially severe in young plantings.
Typical Symptoms of Mal Secco
Symptoms include vein chlorosis (Figure 1) and wilting of leaves, followed by twig and/or limb dieback. The vascular system in an affected tree is often clogged by gum and fungal mycelium. The wood of infected limbs often shows red or yellow streaks, which are diagnostic for the disease (Figures 2 and 3). Infections in the canopy move downward slowly, while infections at the base of the tree may move upward rapidly. Symptoms apparently are associated with toxins and cell-wall degrading enzymes released by the causal fungus. The affected trees quickly decline and die.
Orange-reddish discoloration of an orange limb affected by mal secco disease. Credits: Reprinted with permission from Figure 6 of Q. Migheli, et al., “Mal Secco Disease Caused by Phoma tracheiphila: A Potential Threat to Lemon Production Worldwide,” Plant Disease (2009) 93: 852-67. Courtesy of APS Press.
[Click thumbnail to enlarge.]
Transmission of Mal Secco
The fungus forms a spherical or inversely pear-shaped pycnidium (an asexual fruit body) on twigs, leaves, and fruit. Local spread of P. tracheiphila is by rain-dispersed pycnidiospores (asexual spores) produced from pycnidia (singular pycnidium). Spores can penetrate leaves directly, but infections typically occur through wounds that may be caused by hail, frost, harvesting injury, or pruning. Infections can also occur through exposed roots. Most infections occur during cool periods with rainy weather. Spread within the xylem of the infected trees is associated with the movement of spores. Long distance spread usually occurs through the movement of infected nursery stock.
Detecting Mal Secco in the Field
The main diagnostic field symptom is that wood of affected limbs has a reddish-yellow stain (Figures 2 and 3). This symptom can be seen by making a tangential cut into an affected limb. Isolation and pathogenicity tests can confirm the identity of the causal agent. Serological and molecular assays have also been developed to test for mal secco.
Control of Mal Secco
Long distance spread of mal secco can be prevented by using disease-free nursery materials. Pruning dry twigs and limbs can decrease the rate of spread into the trunk, within affected trees, and to adjacent trees. Removal of suckers and root sprouts reduces the chance of infection in the trunk and main scaffolds. Wounding should be avoided during rainy periods. In Italy, copper-based fungicides are effective for controlling mal secco if applied every 2–4 weeks between October and February. Lemons that are tolerant of mal secco have been identified, but they lack the market appeal of susceptible cultivars. Pre-infection with either citrus exocortis viroid or a hypovirulent isolate of P. tracheiphila has been shown to reduce development of mal secco symptoms.
What Can Growers Do?
While mal secco is a serious problem for certain citrus areas in the Mediterranean, its failure to spread to other regions suggests that climatic conditions limit its distribution or that it is not easily spread to new areas. Better information on the potential susceptibility of Florida cultivars, especially newer releases, would be useful to assess the future threat.
Preventing mal secco from entering Florida is much easier than trying to eradicate or control it. Any citrus-propagating materials must be introduced through the Florida Department of Agricultural and Consumer Services, Division of Plant Industry to ensure healthy plants and the economic viability of the Florida citrus industry.
Footnotes
This document is PP290, one of a series of the Plant Pathology Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Original publication date February 2012. Visit the EDIS website at http://edis.ifas.ufl.edu.
K.-R. Chung, associate professor, Plant Pathology Department, Citrus Research and Education Center, Lake Alfred, FL; Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, FL 32611.
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