Mai Secco of

J. A. Menge Department of University of California, Riverside, CA 92521

Historical

Mai Secco, meaning dry disease, was first recognized on the Greek islands of Chios and Paros in 1904, It spread to Sicily by 1918 and then rapidly throughout the Mediterranean area. Its distribution is still expanding since it reached Spain as recently as 1978.

Causal Agent and Distribution

Mai Secco is caused by tracheiphila Petri (Kane. & Ghik.) formerly known as Deuterophoma tracheiphila. The disease appears to be confined at present to Mediterranean areas. It is known from Greece, Sicily, Cyprus, Italy, Turkey, Israel, Syria, Lebanon, Russia, Algeria, Tunisia, and Spain.

Hosts Affected

All citrus species and hybrids and some closely related genera in the family Rutaceae are affected by Mai Secco. Eureka , , , Bearss lime. Sour , , and Savage are very susceptible. Sweet orange, Monachello and Santa Teresa , citrus volka- meriana, and Palermo mandarin proved to have a high degree of resistance.

Symptoms

The most common symptom of the disease is a sudden wilting and drying of leaves and twigs, often on one side or section of the tree. Symptoms may appear in only a few branches, then spread to larger branches, and finally to the stem and the whole tree may die within a year or two. Young freshly emerging leaves are chlorotic in the spring. A slanting cut through young green branches reveals internal S3nmptoms con¬ sisting of a pinkish or reddish discoloration of the wood. At later stages, this discoloration changes, in succession, to rusty orange, brown, and black. Less virulent strains of the pathogen result in lighter staining in the wood. Following death of the woody tissue, the invades the bark causing it to become a lead gray color. Dark pycnidia appear in these areas. Fruit on diseased branches are readily invaded and may show some vascular discoloration without external lesions.

Epidemiology

Infections under field conditions arise from pycnidiospores liberated from pycnidia which are produced on dead twigs, leaves, or fruit under conditions of high humidity. Germination requires 40 hrs. of moist conditions at 15-16°C. Germ tubes enter leaf tissue through the stomata. However, the most severe infections occur through wounds such as those made by wind, hail, cultivation, or fertilization. Since the disease spreads upward through the vascular tissue much faster than it does downward, root infections and stem

1 infections are usually lethal whereas leaf infections are of less consequence. Windblown rain and low temperature aggravate the disease. Primary infection normally takes place between November and February during the rainy season when the plant is partially dormant. Symptoms appear the following spring.

Identification

Identification of Mai Secco is fairly straightforward. Symptoms are distinct. In addition, wood infected with Phoma tracheiphila shows a pink or yellow-green discoloration in 1% NaoH or KOH. The fungus itself is charac¬ terized by the presence of brown or black pycnidia 30-105 ym in diam. with a neck 40-50 ym long. The small bacillus-shaped spores are 1.0-4.0 ym long by 0.8-1.5 ym wide.

Control

Control measures to date are unsatisfactory. Resistant rootstocks should be used where feasible. Deep cultivation, fertilization, and other cultural practices which could wound trees during the infection period should be avoided. Fallen leaves and trunk debris should be removed and burned to reduce inoculum. Diseased shoots and branches should be pruned during the summer. This removes the organism and prevents spread through the vascular system as well as removing inoculum for the next season. Control can also be gained by spraying with copper fungicides or benomyl during the infection period.

Economic Importance

Mai Secco is a highly destructive disease and citrus orchards have been severely damaged wherever the disease occurs.

Potential Hazard to the U.S.

This disease is ideally suited to a climate like that in southern California. If introduced, it is fairly certain to cause severe damage throughout U.S. citrus growing regions.

Recommendations for Future Action

Mai Secco represents an extreme hazard to U.S. citrus. All possible pre¬ cautions should be made to prevent introduction of this organism into the U.S. A contingency plan of action should be developed in the event that Mai Secco should become established in the U.S.

2 References

Fawcett, H. S. 1936. Citrus diseases and their control. McGraw-Hill, New York, 656 p.

Klotz, L. J. 1956. Mai Secco disease of citrus. California State Dept. of Agric. Bull. 45: 234-237.

Knorr, L. C. 1965. Serious diseases of citrus foreign to Florida. Florida Dept. Agric. Bull. 5, 59 p.

Laborda, E. and A. Sanchez. 1974. The Mai Secco on citrus in Spain. Bol. R. Soc. Esp. Hist. Nat. Secc. Biol. 72: 29-34.

Luisi, N. , V. De Cicco, G. Cutuli, and M. Salerno. 1979. Analysis of the pathogenicity of Phoma tracheiphila (Petri) Kane. & Ghik. on some citrus species and . Phytopathologia Mediterranea 18: 162-165.

Ruggieri, G. 1948. Fatlori che condizionano o contribuiscono alio sviluppo del "Mai Secco" delgi atrumi e metodi di lotta contro il medesimo. Ann. della Sperimentazione Agraria. N. S. 2(2): 1-51 (Foreign Agric. Circ. Trans. July 28, 1950, 21-63 pp.).

Solel, Z. 1976. Epidemiology of Mai Secco disease on lemons. Phytopath Zeitschrift 85: 90-92.

Solel, Z., J. Pinkas, and G. Loebenstein. 1972. Evaluation of systemic fungicides and mineral oil adjuvants for control of Mai Secco disease of lemon plants. Phytopathology 62: 1007-1013.

Solel, Z. and Y. Oren. 1975. Outbreak of Mai Secco disease in Israel on nor¬ mally tolerant citrus cultivars. Plant Dis. Rept. 59: 945-946.

Solel, Z. and P. Spiegel-Ray. 1978. Methodology of selection of lemon clones for tolerance to Mai Secco (Phoma tracheiphila). Phytoparasitica 6: 129-134.

Solel, Z., D. Sandler, and A. Dinour. 1979. Mobility and persistence of car- bendazim and thiabendazole applied to the soil via drip irrigation. Phytopathology 69: 1273-1277.

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