First report of stem rot of rosemary caused by Sclerotinia sclerotiorum in the United States

Rosemary (Rosmarinus officinalis L.) is a culinary and ornamental herb grown commercially in Oregon. In October, 2000, young plants growing in a greenhouse with branch dieback were observed with grey lesions on the lower stem, wilt of the foliage, and entire plant necrosis. Affected plants had physical injury to the stems from fungus gnat larvae. In 2002, 2-year old, field-grown plants with similar symptoms were submitted for diagnosis. These plants were taken from a 0.25 Ha field in which 50% of the plants were affected and both plants submitted had basal stem wounds. Isolations from both the greenhouse and field-grown plants yielded Sclerotinia sclerotiorum, which was identified on the basis of morphology.

Inoculum was generated by placing sterile toothpicks on the surface of colonies in potato dextrose agar culture to allow mycelium to grow over and into the wood, growing the fungus on sterile wheat seeds for 15 days for colonized grain, or for 12 weeks on sterile wheat for sclerotia production. Ten 3-month old rooted cuttings were inoculated by placing a colonized seed 1 cm away from the stem, 1 cm beneath the surface of the potting mix. Seven plants received sterile wheat seeds. Five plants were inoculated with sclerotia placed similarly. To determine if wounding was required for infection, five plants were inoculated by inserting a colonized toothpick approximately 3 mm deep into the stem 1 cm above the soil line. Five control plants were similarly pierced with sterile toothpicks. All inoculated plants (all treatments) were misted to run-off and bagged in plastic for 3 days to maintain high humidity. The bags were removed after that time.

Of the plants inoculated with the colonized grain, two plants had died after 10 days, and the fungus was recovered from them. The remaining were asymptomatic and did not yield Sclerotinia upon isolation. None of the plants inoculated with the sclerotia became infected. Four days post-inoculation the toothpick-inoculated plants were showing initial symptoms of foliar wilt, and by 7-10 days post-inoculation symptoms similar to those originally observed had fully developed. S. sclerotiorum was recovered from all toothpick-inoculated plants, but not from control plants, which remained asymptomatic.

There has been one previous report of S. sclerotiorum causing disease in rosemary, from Ooty, India (Mohan, 1994). This appears to be the first report of this disease in Oregon and in the United States.