- A study was conducted of the influence of various climatic and
soil factors on the development of onion pink root disease and of the
influences of temperature, light, pH, carbon and nitrogen sources,
and vitamins on the growth in vitro of an Oregon isolate of the pink
root fungus, Pyrenochaeta terrestris (Hansen) Gorenz, Walker,
Eleven onion lines were planted in eastern and western Oregon
to determine whether the relative resistance to pink root among them
would vary during the growing season and with different climatic
conditions. Varietal resistance remained generally stable among
the different lines.
Ten onion lines were grown in mucky peat and silty clay loam
soils in the greenhouse to determine the influence of soil moisture
and soil type on severity of pink root. Onions grown in soil held at field capacity had less pink root than those in dryer soils. Pink
root ratings were similar among onions grown in both soils.
Lanstan, Vorlex, Telone, Pictel, Morton Soil Drench, SD 345,
chloropicrin, Mumfume, Phaltan, E.P. 230, and E.P. 201 were
tested in the field for their effect on pink root. In some fields
Vorlex, chloropicrin, Mumfume, E.P. 230, and E.P. 201 increased
onion yields from 30 to 75 percent. None of these fumigants
noticeably reduced pink root, however. Telone controlled stubby
root nematode infection but did not reduce pink root.
On malt extract-yeast extract agar P. terrestris grew fastest
at a temperature of 30 C, with the greatest increase in growth rate
between 15 and 20 C. There was no growth at 5 C.
Continuous light, alternating darkness and light, and brief
exposures to germicidal ultraviolet light (2,500 A[superscript °]) did not noticeably
affect the growth rate of the fungus.
Growth of P. terrestris increased the pH of glucose-asparagine
and sucrose-potassium nitrate liquid media. On both media the
fungus grew well over a pH range from 6 to above 8 but poorly at
a pH of 4 or less.
Sorbose inhibited growth of P. terrestris. Maltose, galactose,
glucose, fructose, sucrose, lactose, and starch in asparagine
liquid and agar media each supported good growth of the fungus, as
did glycine, potassium nitrate, and urea in glucose agar.
The addition of thiamine, biotin, inositol and pyridoxine to
sucrose -asparagine agar did not noticeably affect P. terrestris
Higher temperatures coincided with greater pink root infection
in the field.