Sept, a», 1923 
Bacterial Leaf spot of Clovers 
489 
The most rapid spread of bacterial spot, as shown by field observa¬ 
tions, occurs when there is an abundance of rain or dew. At these 
times conditions are most favorable for the spattering of the bacterial 
exudate to adjacent healthy leaves of plants, and opportunity is given 
for the bacteria to gain entrance to the leaves by means of the surface 
film of moisture. 
Aside from rain and dew as agents of dissemination of bacterial leaf- 
spot, there is considerable observational evidence that certain leaf-eating 
insects, especially the larvae of the clover leaf weevil, Phytonomus 
punctatus, are responsible for its spread. Initial infections not uncom¬ 
monly occur at the places where the leaves are injured by the feeding of 
these insects. In experimental feeding trials, clover leaf weevils avoided 
in every case eating other than the healthy tissues of abundantly spotted 
leaves offered them. The fact, however, that perforations made by 
weevils are the loci of infection indicates insect carriage. Many other 
plant pathogens whose normal mode of entering is through stomates 
are known to gain entrance also through wounds. 
SUMMARY 
(1) A hitherto undescribed bacterial leaf spot disease has been observed 
on several species of clover, including Trifolium pratense , T. medium , T. 
repens , T. repens var. latum , T. hybridum , T. incarnatum, T. alexandrinum , 
and T. pannonicum . It is known to occur in Wisconsin, Iowa, Indiana, 
Virginia, Maryland, and North Carolina, and is probably widely prevalent. 
(2) Leaves, stems, stipules, petioles, and flower pedicels are known to 
be involved, but lesions have not been observed on the floral organs. 
(3) The spots may appear at any time throughout the growing season. 
The lesions on the leaves are at first minute, translucent dots which 
enlarge and become, at length, irregular, blackish-brown areas. These 
areas have a translucent border and the surrounding tissues are yellowish 
green. Mature leaves are perforated and frayed, due to the drying and 
falling of portions of the affected tissues. 
(4) Under favorable moisture conditions, a milky white bacterial 
exudate is formed on the lower leaf surface. On drying, this becomes a 
delicate incrusting film. 
(5) Bacterial leafspot is caused by an organism which is herein 
described as Bacterium trifoliorum , n. sp. It forms whitish colonies on 
nutrient agar, is flagellate, and forms acid from dextrose and saccharose. 
According to the Descriptive Chart, its group number is 212.2322023. 
(6) With the type strain from Wisconsin, infection was secured only 
on the red clovers, but with the strains from North Carolina successful 
reciprocal inoculations have been made on the red, white, and alsike 
clovers. The Virginia strains also cross-infect successfully. The para¬ 
site is intercellular and apparently enters chiefly through the stomates. 
(7) Field observations indicate that the dissemination of the disease 
is accomplished through the agency of rain or dew and of leaf-eating 
insects. 
(8) It seems very probable that the organism is disseminated with the 
seed and that such contaminated seed in new plantings are in conse¬ 
quence the primary loci of infection. 
