1090 
Journal of Agricultural Research 
Vol. XXX, No. 11 
its potential value as a rootstock for 
nursery use, because scab has a marked 
stunting effect upon very young trees 
of susceptible varieties. 
The Sour and Bittersweet oranges 
(Citrus aurantium) are severely at¬ 
tacked by scab; the varieties Myrtle 
Leaf and Willow Leaf are moderately 
attacked, but the disease has not been 
observed on the Bergamot and 
Otaheite forms. 
The common or sweet Florida orange 
of commerce ( Citrus sinensis ) is highly 
resistant to scab. This species is 
affected so rarely that for all practical 
purposes it may be considered immune. 
A trace of scab has been observed a 
very few times on each of these follow¬ 
ing varieties—Florida Seedling, Lue, 
Maltese, Mediterranean Sweet, Pine¬ 
apple, and Washington Navel. The 
diagnoses in these cases were verified 
by cultures from the affected parts. A 
scab-like blemish of undetermined 
cause has been observed occasionally 
on the sweet orange, but microscopic 
and cultural studies have consistently 
failed to reveal the scab fungus in the 
affected tissue. 
The kid-glove oranges ( Citrus nobilis ) 
on the average at least, are as likely to 
develop scab as the grapefruit. The 
King variety is very susceptible, the 
Satsuma is distinctly less severely at¬ 
tacked, while the Cleopatra mandarin 
appears to be immune. Because ol its 
immunity to scab and because it de¬ 
velops an excellent root system, the 
Cleopatra mandarin is gaining popu¬ 
larity as a rootstock in Florida. 
The calamondin ( Citrus mitis) is 
about as severely affected as the King. 
The pointed-leaf form of Papeda 
(■Citrus hystrix ) is also severely at¬ 
tacked by scab, while the round-leaf 
form has not developed this disease. 
Thus it seems that in the several 
groups susceptible to scab infection 
there is at least one commercial mem¬ 
ber on which scab occurs, though Citrus 
sinensis and Fortunella margarita, for 
practical purposes, are immune. 
CITRUS HYBRIDS 
Observations have been made on a 
rather large range of citrus hybrids and 
the degree of infection recorded for 
them is tentative and unless otherwise 
stated refers to leaf infections only. 
Many of these forms most likely will 
never be grown extensively, and only a 
few of them are now distributed 
through the citrus belt of Florida. 
For these reasons the relative suscepti¬ 
bility to scab can not be determined 
accurately at this time nor possibly 
in the near future. Table II gives the 
impressions gained from inoculations 
and numerous observations on the 
degree of scab infection on some citrus 
hybrids. 
The data in Table II show that when 
both parents of the hybrid are attacked 
by scab the progeny in general is more 
likely to be affected by that disease 
than when only one parent is attacked, 
and a hybrid resulting from the cross¬ 
ing of two scab-resistant forms is 
likely to inherit the scab resistance of 
its parents. With but few exceptions, 
citrus hybrids are at least as severely 
attacked as the most susceptible 
parent. The only hybrid under ob¬ 
servation with a scab-susceptible parent 
that has thus far been scab free is 
the Faustrimedin, a plant that does 
not resemble the Calamondin in any 
of its gross characters. It seems from 
the data in Table II that liability 
to scab attack is transmitted from 
susceptible parents to the first-gen¬ 
eration progeny. 
CITRUS SCAB OUTBREAKS 
Citrus scab is very erratic in its out¬ 
breaks. Tender growth is the most 
highly susceptible. The outbreaks vary 
not only from season to season, de¬ 
pending on weather conditions, but 
with individual trees. The variation 
with different trees seems to be due to 
the fact that under Florida conditions 
a citrus tree may start growth as 
much as two weeks or more before 
the surrounding ones and even on 
one side of a tree new growth may 
sometimes be a week or more slower 
in starting than on the other. This 
disease also varies from district to 
district. The grapefruit and lemon 
in the Rio Grande Valley are affected 
by scab much less than in Florida. 
In Alabama the Satsuma and the 
Trifoliata orange are much more 
severely attacked than the same 
varieties are in Florida. Peltier and 
Frederich 6 report that in southern 
Alabama scab has not been observed 
on the Orangelo, Siamelo, or Siamor. 
The disease has been observed on 
these hosts in Florida. The Cit- 
rangequat seems to be more severely 
attacked in Alabama than in Florida. 
The difference in degree of scab out¬ 
break on a host in the several States 
seems more likely to be due to climatic 
conditions while the host is at a 
susceptible stage of growth rather than 
to fluctuating differences in inherent 
susceptibilities of the hosts or to altered 
virulence in strains of the pathogene. 
6 Peltier, G. L., and Frederich, W. J. relative susceptibility of citrus fruits and hybrids to 
cladosporium 3 tri massee. Jour. Agr. Research 24 : 955-959. 1923. 
