Structure and Composition of Citrus Leaves Affected with Mesophyll Collapse.

Introduction A pathological condition not infrequently observed in leaves of orange trees as yellow, translucent, sunken areas, has been called "mesophyll collapse" by FAWCETT (6). Neither bacteria nor fungi seem to initiate this disorder of the leaves, though they may invade the weakened and collapsed tissues secondarily. The disorder has therefore been regarded as a physiological disturbance caused by insects and by edaphic and climatic factors, or by edaphic or climatic factors alone. FAWCETT (6) first found mesophyll collapse in the coastal districts of southern California. In a series of papers, FAWCETT et al. outlined the distribution of mesophyll collapse in South America. They found it in the coastal areas of the state of Rio de Janeiro, Brazil (11), but did not find it two hundred miles in the interior, in the state of Minas Geraes (12). It was also found fairly near the coast, in the states of Pernambuco (7) and Bahia (8), and in Sao Paulo as far as 40 miles inland and at elevations as high as 2,500 feet above sea level (8). None was noted in the Pitangueires area, some two hundred miles from the sea coast; in the state of Rio Grande do Sul, however, it was found near the coast, at Montenegro (8). It was noted at Uruguayana, somewhat inland, near the Uruguay River (8), as well as at Concordia, Argentina, also near the Uruguay River. These investigators, however, founid no mesophyll collapse in any other parts of Argentina (9). In Paraguay, mesophyll collapse was found at Asuncion, near the Paraguay River (10). The writers have made observations in North America and have noted the occurrence of mesophyll collapse in the coastal districts of the United States and Mexico. We have found it in the coastal districts of Florida, near Vero Beach, and in the coastal valleys of California. It has been observed in Arizona2 and it probably exists, also, in some parts of Texas near the gulf. Our survey in California has shown that the disorder is present in Ventura, Santa Barbara, Los Angeles, San Bernardino, Riverside, Orange, and San Diego counties. Its general distribution in California is shown in figure 1. This leaf condition is most readily recognized when the mature leaves are examined by transmitted light. It will then be observed that yellow, translucent areas lacking chlorophyll occur principally in the central portion of the blade, on one or both sides of the midrib (plate I, fig. 2). By reflected 1 Paper no. 484, University of California Citrus Experiment Station, Riverside, California.