DISEASE PROBLEMS

DISEASE PROBLEMS

XYLOPOROSIS

The principal disease in groves of the Gaza Strip is xyloporosis. This will come as no surprise to those familiar with citrus pathology at the eastern end of the Mediterranean where Palestine sweet lemon has long been employed as a rootstock. The first observation and the original description of xyloporosis were made in Palestine in the early thirties (8).

Today it is mainly the old groves of Palestine sweet lemon that are affected by xyloporosis. None of the trees on sour orange, which now constitute 80 percent of the acreage, show ill effects from the virus.

Of the 20 percent of the trees that continue on Palestine sweet lemon, many are in a marginal state of productivity. Not all of the depauperate growth, however, can be attributed to xyloporosis. Entire blocks of Shamouti orange trees on Palestine swell lemon have been found to exhibit a uniform depression of growth, with only scattered trees showing the greater degree of stuntiq and the characteristic pitting and pegging d xyloporosis. The general "running-ouf effect, which disposes some growers to the belief that the natural life span of citrus trees is only 25 years, it gets that sweet lime is liable to troubles other than xylopo rosis, and that these troubles are more serious than xyloporosis itself. The fact that sour orange does better in the heavier, wetter soils of the Strip points to the possibility that Phytophthora fungi may be destroying roots of the more susceptible sweet lime (1). Also involved may be a decline that correlates with the presence of vascular honeycombing in the scion (i.e., Shamouti) portion of the trunk immediately above the bud union. Thisss gum-free inverse pitting is found, in the absence of xyloporosis pitting, in the sweet lime stock, and recalls trouble pre­viously described from Egypt by Nour-Eldin and Childs (5). In some blocks of Shamoutis on sweet- lime rootstocks, xyloporosis has been found to affect as high as 15 percent of the trees. In other blocks, many of the affected trees have been removed, so that maximum figures for damage due to xyloporosis are obliterated. More ingenuous than ingenious is the occasional practice of putting two sweet ­lime-rooted trees in each planting hole, thus hopefully providing at least one tree that will not be affected by xyloporosis.

FVOEA

In a block of ten-year-old Clementine mandarin trees on sweet lime rootstock, approximately 10 percent of the trees were found to be in a marked state of stunting and decline. This condition was correlated with a honeycombing (inverse pitting) and gumming in the vascular region of the trunk, beginning at the bud union and progressing upward for a distance of several feet into the framework of the tree. Symp­toms, both external and internal, resemble those found in the fovea disease that affects Murcotts in Florida (3). It is still not known whether fovea _.is a varietal response to xyloporosis or cachexia virus or whether another virus is involved. Five out of the six trees examined showed no xyloporosis pitting in the sweet lime portion of the trunk; this would suggest that xyloporosis and fovea are indeed separate troubles.