322 
Joumal of Agricultural Research 
Vol. XXVIII, No. 4 
every leaf, stem, and nut growing on the susceptible trees becomes badly infected 
and the orchards become public nuisances, spreading the disease to near-by 
orchards in all directions. 
The distribution of pecan scab is very noticeably associated with climatic 
conditions. With the exception of the humid Coastal Plain region, Texas ordi¬ 
narily is practically free of the disease. Therefore, in that State, Delmas, one 
of the most susceptible varieties, is strongly recommended for planting. The 
disease is most destructive and attacks the widest range of varieties in the humid 
coastal region of Louisiana, Mississippi, Alabama, Georgia, and Florida. 
VARIETAL SUSCEPTIBILITY 
At present there is a wide variation in the degree of susceptibility of the different 
cultivated pecan varieties to scab. Some varieties are so severely attacked 
during seasons favorable for the dissemination and propagation of the fungus 
that the entire crop may be destroyed. Other varieties are so highly resistant 
that they may at present be considered almost immune. Wild or planted seed¬ 
lings show the same variation in resistance. 
It seems probable that, in soipe instances, the distribution of pecan scab and 
apparent differences in the relative susceptibility of the same varieties growing 
in different localities can be attributed to different strains of the pathogen. If 
different strains do not exist, then how can one explain why two susceptible varie¬ 
ties, as Delmas and Schley, can grow in contiguous rows for several years, the 
Delmas variety being badly infected but the Schley free of the disease? Then 
after an interval of a few years, the Schley also becomes infected, at first only 
slightly so, but later severely. Possibly, the Schley strain if there is such, was 
present in the Schley orchard during the time of its apparent absence, but in¬ 
creasing slowly, finally became abundant in the Schley orchard. It is possible 
that the strain on the variety Schley may have developed from the Delmas 
strain by some process of adaptation. 
A classification of various cultivated varieties of pecans according to the 
degree of their susceptibility is not easy, in as much as the susceptibility of each 
variety varies in widely separated localities. What may be considered at present 
as a highly resistant variety in one locality may be a susceptible one in another 
place. Generally speaking, however, Delmas is the most susceptible variety 
followed by the varieties Georgia, Alley, Van Deman, Schley, Pabst, Mobile, 
Success and Moore, in the order named. The following varieties, while prob¬ 
ably not entirely immune, are highly resistant: Teche, Curtis, Moneymaker, 
Russel, Stuart and Frotcher. The Texas varieties^are purposely omitted from 
the list as ordinarily pecan scab is not an important disease in that State. Some 
Texas varieties, notably, the San Saba, are highly susceptible when planted in 
the humid region of the southeast United States. 
DESCRIPTION OF THE DISEASE 
ON THE LEAVES 
On the leaves the infections first appear as elongated spots on the large vein 
on the underside of the pinnae or on the rachis and petiole, Plate 1, A. A few 
days later, spots may be noticed on the underside of the smaller veins also. The 
lesions on the small veins are at first elongated but later diffuse irregularly into 
the healthy tissues, forming more or less circular spots. Some infections appear 
t<5 have taken place on the upper surface of the leaflets. Apparently not all 
spots begin on the veins, since some appear on the green parts lying within the 
netwo rk of the vascular system. The spots [are at first olive brown in color, but 
turn almost black with age. They are quite conspicuous on the leaves and 
