Report on the Agricultural Experiment Stations, 1953 Page: 98
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98 REPORT ON EXPERIMENT STATIONS, 195 3
to stalk rot being least in the checks, significantly higher in plots
inoculated with Gibberella, and highest in plots inoculated with
Diplodia. A striking result was the development of natural gibberella
rot infections at above-ground nodes in 75 percent of the plants
with clipped leaves, compared to only 17.3 percent of such infections
in plants with normal leaves.
A histological study of tobacco black shank infections, reported by
the North Carolina station, has shown that the causal organism
(Phytophthora parasitica var. nicotianae) freely penetrates the roots
of both susceptible (402) and resistant (Dixie Bright 101 and Dixie
Bright 102) varieties. In the susceptible variety the pathogen rapidly
ramifies through living host cells and colonizes abundantly in
the invaded tissues. Impoverishment of the parasitized host cells
quickly follows, resulting in the depletion of their contents. Invasion
by the fungus continues until the entire root system is destroyed and
the plant dies. In the resistant varieties, however, the epidermal cell
entered by the fungus and those adjacent to it suddenly died and
collapsed. Owing to this hypersensitive host cell response, the invading
pathogen makes only slow, feeble growth and often fails to
become established. It was found that this hypersensitive response
was characteristic only of epidermal and cortical tissues of resistant
roots and that, if the pathogen were in some way introduced into the
pith or vascular system of such roots, pathogenesis would proceed in
a manner similar to that noted in susceptible plants. This may explain
how root injuries caused by parasitic nematodes or other agents
may predispose resistant plants to black shank and increase the incidence
of the disease in resistant varieties.
The Virginia station found that soil fumigation and rotation reduce
pathogenic nematodes and give greater returns from the black
shank resistant varieties. It was also observed that moderately resistant
varieties with more desirable qualities can be used if the soil
is fumigated, or in a rotation.
The Virginia station estimates that in 1952 the tobacco growers in
the State lost over 12 million dollars from tobacco diseases. Black
shank caused the heaviest loss, estimated at over 5 million dollars,
and root knot nematode caused damage amounting to over 4 million
dollars. The value of the State tobacco crop, all types, was nearly
85 million dollars.
The Connecticut Agricultural Experiment Station (New Haven)
discovered that downy mildew of tobacco seemed to get started from
local sources of inoculum. The mycelium and conidia (spores) of the
mildew fungus overwinter on old tobacco leaves and provide a local
source of inoculum in the ensuing year.
Experiments at the Puerto Rico University station did not confirm
the general farmer belief that the mosaic-susceptible sugarcane variety
B. 34-104 could produce more sugar than the currently known commercial
varieties, and that this variety recovers from the disease.
Once the canes of B. 34-104 were infected with the common sugarcane
mosaic prevailing in the Island, they did not recover from the disease
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United States. Office of Experiment Stations. Report on the Agricultural Experiment Stations, 1953, book, 1953; Washington D.C.. (digital.library.unt.edu/ark:/67531/metadc5989/m1/100/: accessed July 28, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.