une i6, 1923 
Susceptibility of Citrus Fruits 
957 
Poncirus trifoliata varies from slightly susceptible to susceptible, de¬ 
pending on the season. As a rule, very little scab occurs after primary 
infection takes place on the unfolding biids and the young shoots in the 
spring. 
The round leaf form of Citrus hystrix DC. which is susceptible to 
canker is only occasionally attacked by scab, while the pointed leaf form 
which is semi-resistant to canker is very susceptible to scab. Primary 
infection is usually severe on young leaves and shoots, although the 
plants present a scabby appearance throughout the growing season. 
The fact that Citrus hystrix, native of the Philippine Islands, where no 
scab has ever been found, is susceptible under our Gulf Coast conditions, 
leads one to assume that Cladosporium citri is unable to persist under 
Philippine conditions, which is one reason why scab has never been re¬ 
ported from the islands, rather than the fact that scab has never been 
introduced. 
The Ichang lemon, possibly a natural hybrid between a lemon and 
pummelo, appears to be more susceptible to scab than the sour and 
commercial varieties of lemons. 
The behavior of the plants belonging to the Citrus grandis group in 
their susceptibility to scab is quite variable, ranging from nonsuscepti- 
ble to very susceptible. Whether there is a difference in the varietal re¬ 
sistance is not known. The fact that plants of all ages, conditions, and 
types were employed makes it doubly hard to draw any conclusions. 
Judging from observations of other investigators, the grapefruit has only 
within recent years been reported susceptible to scab. On the whole, it 
can be said that the ordinary Florida varieties, as represented by Duncan 
and Sullivan, are only slightly susceptible; the Florida Shaddock, very 
susceptible; the Indian and/ Chinese pummelos, susceptible; and the 
Siamese pummelos, slightly" susceptible. The Hirado Buntan pummelo 
has never developed scab in the field. While there is a decided differ¬ 
ence in scab susceptibility of the various plants in the C. grandis 
group, we do not know whether the differences are varietal or due tc 
their individual reaction to environmental conditions which in turn in¬ 
fluences scab susceptibility. 
No scab has ever been noted on any of the plants belonging to the 
Citrus sinensis group. T^hese observations are in common with those 
made by most investigators. Stevens (7) reports the Due Gim Gong as 
attacked by scab in Florida, and it is the only variety of sweet orange so 
recorded in America. Earle (i) in Porto Rico finds that some of the 
round oranges are occasionally attacked by scab. Reinking {6) also 
reports a plant of the C, sinensis group subject to scab in China. On the 
whole, it can be stated that the sweet orange group, with an occasional 
exception, is nonsusceptible to scab. 
Of the C. nohilis group, both the King orange and Satsuma are 
slightly susceptible to susceptible, depending on weather conditions. 
Most of the scab occurs during the early spring on the young leaves and 
' small fruits. During late seasons, such as prevailed in 1915 and 1920, Sat- 
sumas are badly attacked by scab, resulting in considerable damage to 
the fruit. The Cleopatra tangerine, which is semiresistant to canker, 
evidently does not scab. It is one of the few plants promising as canker 
resistant wfircH'is'not attacked by scab. 
Citrus mitis Blancho is another example of a plant native to the 
Philippine Islands which is very susceptible to scab under our txulf 
Coast conditions. Scab is usually quite severe in the early spring on the 
