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Photoperiod Influence on the Susceptibility of Sunflower to Sclerotinia Stalk Rot. R. G. Orellana, Research Plant Pathologist, Applied Plant Pathology Laboratory, Beltsville Agricultural Research Center, Agricultural Research Service, U.S. Department of Agriculture, Beltsville, Maryland 20705; Phytopathology 65:1293-1298. Accepted for publication 10 June 1975. DOI: 10.1094/Phyto-65-1293.

Photoperiod affected reaction of the annual sunflower Helianthus annuus to Sclerotinia sclerotiorum, a soilborne pathogen that causes stalk and head rot. Cultivars grown in the growth chamber at approximately 1,900 lux and 22/20 C day/night temperatures under short days on an 8- and 14-hour photoperiod were susceptible. In contrast, cultivars grown under long days on an 18- and 24-hour photoperiod were tolerant. Annual sunflowers grown in the greenhouse were highly susceptible, in both the seedling and adult stages. The susceptibility of the annual sunflower was associated with the proneness of succulent hypocotyls to fungal infection in response to short photoperiods. Tolerance in H. annuus was associated with enhanced growth and lignification in response to long photoperiods. The perennial H. tuberosus and interspecific hybrids H. tuberosus × H. annuus and H. tuberosus × H. annuus × H. strumosus were resistant to stalk rot, irrespective of photoperiodic treatment.

Additional keywords: Whetzelinia sclerotiorum.