Apr. 26,1924 Pecan Scab and Sources of Early Spring Infections 
323 
especially so on the light yellow green of the young leaves. The infections on 
the leaf may be few or many, depending upon the variety, the abundance of 
sources of infection and the weather conditions. Leaves become more immune 
as they reach maturity and as a rule, are only slightly susceptible after they 
change from the light yellowish-green of the young stage to the later deep green 
color. The control of the disease would be much simplified if the pecan did not 
have the habit of putting out new leaves and twigs after the initial spring growth. 
A heavy second growth of foliage, as a rule, is produced about the 1st of July or 
soon after the usual summer rains commence. While on the light growth of 
new leaves during May and June new infections continue to occur thus forming 
new sources of infection, it is the new infections on the midsummer second growth 
of leaves which serve greatly to increase these sources of infection. A heavy 
increase of the disease on the nuts when they are about one-half grown as a rule 
follows the severe spotting of the second growth of foliage. Ordinarily, the 
development of the midsummer second growth of foliage extends over a period 
of four to six weeks. During this time new leaf surfaces are continually being 
exposed to infection. A protective spray may be applied today but to-morrow 
new leaflets are exposed and unprotected. 
The damage done to the leaves as a whole is of minor importance, though 
considerable defoliation, especially in nurseries, sometime occurs (PI. I, G), 
and heavily infected trees shed their leaves early in the autumn. Several 
infections on the veins of the ventral side of the leaflets cause them to curl 
downward as though injured by aphids. The punctures of aphids, often found 
feeding on the veins during early spring, are considered by Waite s as points of 
entrance for the pathogen. Possibly the greatest damage done to the leaves is 
due to the interference with their normal functioning caused by the partial 
girdling of the petioles from numerous infections near their bases. 
ON THE NUTS 
Narrow, elongated spots frequently appear first on the young nuts along the 
creases which run parallel to the long axis of the nut midway between the 
sutures of the pericarp, and which correspond morphologically to the veins of 
the leaflets (PI. 1, C and D). Lesions are quite small at first and slowly 
enlarge, some attaining a diameter of about 12 mm. Most spots, however, 
cease to enlarge after they are about 3 mm. in diameter. The scab spots on 
the nuts may be either olive-brown or gray when first visible. Those infections 
which are at first gray later change to an olive-brown with a gray border. All 
scab spots, regardless of shape or color, possess a more or less jagged, irregular, 
and indefinite outline. 
The greatest damage is done to the nuts, which may become undersized, 
faulty, or completely destroyed. The diseased areas are rather superficial, 
extending only slightly below the epidermis. The affected tissues become 
black, hardened, and somewhat cracked, but show no tendency to decay (PI. 
1, D). It is not uncommon, however, for secondary fungi to gain entrance 
through the ruptured surface caused by the scab pathogen and produce deep- 
seated lesions extending down through the exocarp to a depth of 2 to 2i mm. 
The secondary fungi apparently do not enter the husk until the nut is from 
one-half to two-thirds grown. When scab infections occur while the nuts are 
small, or when numerous infections occur during midsummer, the growth and 
development of the nuts are completely retarded, and they either fall before 
attaining their normal size or blacken and hang on the trees all winter (PI. 
1, E; PL 2, D). The death of the nuts often seems to be due to the coincidence 
» Waite, M. B. pecan scab [fusicladixjm effustjm]. Science 33: 77-78. 1911. 
