Bulletin 214, Cotton Diseases in Florida


 LEAF SPOTS
 There are a con-
 siderable number
 of fungi causing
 leaf spots on cot-
 ton, but most of
 them are of small
 or occasional im-
 portance. The
 m'o s t commonly
 observed and im-
 portant of these
 leaf spots is caus-
 ed by an Alterna-
 ria sp. This fun-
 gus attacks cot-
 ton leaves that
 are weakened by
 rust or which
 show bacterial or
 Cercospora le-
 sions. The spots
 are usually ir-
 Fig. 13.-A cotton leaf showing the type of injury regularly circu-
caused by Alternaria sp. Note concentric ringing in
certain of the spots. lar in shape and
 may reach a size
of one-half inch in diameter. The spot is brown in color, of papery
consistency, and is usually marked by a series of concentric rings.
The spores are borne on these spots, sometimes in sufficient
quantity to give a black cast to the spot. By the coalescence
of several spots the whole leaf may become involved and soon
shrivels and falls off. Under moist conditions, defoliation may
be severe; and late in the season the fungus is extremely abun-
dant on the mature leaves and causes them to be shed earlier
than would normally occur. In Fig. 13 is shown a leaf attacked
by Alternaria.
 Another extremely common leaf spot in Florida is that caused
by the conidial stage (Cercospora gossypina Cke.) of Mycos-
phaerella gossypina (Atk.) Earle. The Cercospora leaf spot, as it
is commonly called, is extremely common in Florida cotton fields,
but usually does not cause serious defoliation unless accompanied
by Alternaria. The fungus is capable of attacking uninjured tis-