Mexican sunflower (Tithonia rotundifolia) is a plant in the Asteraceae that is grown commercially as a cutflower commodity and also as a beneficial insectary plant. In June 2012 in coastal California (Santa Cruz County), several fields of organic lettuce (Lactuca sativa) were interplanted with direct-seeded rows of Mexican sunflower (cv. Torch) in order to attract beneficial insects. When approximately 2 to 3 weeks from harvest, lettuce plants began to wilt and collapse. Lettuce crowns were decayed and covered with white mycelium and small (0.5 to 3 mm diameter), irregularly shaped, black sclerotia. These plants were confirmed to have lettuce drop disease caused by Sclerotinia minor (2). In addition, Mexican sunflower plants began to wilt and eventually died. Initial symptoms on crowns and bases of the main stems in contact with soil consisted of a light tan discoloration. These discolored areas turned darker brown, became necrotic, and later were covered with white mycelium and sclerotia that were identical to those found on lettuce. Symptomatic sunflower stems were surface disinfested and small pieces from the margins of necrotic areas were placed into petri plates containing acidified potato dextrose agar. Resulting fungal colonies were white, produced profuse numbers (approx. 39 sclerotia/cm2) of small black sclerotia, and were identified as S. minor. Six-week-old Mexican sunflower plants grown in a peat moss-based rooting medium in 5-cm square pots were used to test the pathogenicity of four isolates. Isolates were grown on cubed and autoclaved potato pieces and resulting sclerotia were recovered and dried (1). For each isolate, 12 plants for each of three cultivars (cvs. Fiesta del Sol, Torch, and Yellow Torch) were inoculated by placing 3 to 5 sclerotia 1 cm below the soil level and adjacent to the plant crowns/stem bases. Sterile sand was placed next to crowns of the control plants. Plants were maintained in a greenhouse at 22 to 24°C. Symptom development was rapid and after 6 to 7 days, inoculated Tithonia plants exhibited brown necrosis at inoculated areas. After 10 days, Tithonia crowns were decayed and plants wilted. S. minor was reisolated from selected necrotic crown and stem tissues. Diseased plants that were not used for reisolations later supported the growth of the characteristic white mycelium and black sclerotia. There were no significant differences between the Tithonia cultivars, and overall disease incidence ranged from 74 to 100%. Non-inoculated plants were asymptomatic. The experiment was repeated and results were similar. In addition, the sclerotia of the four Tithonia isolates were similarly inoculated onto sets of 12 romaine lettuce plants (cv. Green Towers). After 5 to 6 days, all plants developed lettuce drop disease and the pathogen was reisolated. To my knowledge, this is the first report of Mexican sunflower as a host of S. minor. These findings indicate that Mexican sunflower and lettuce are susceptible to the same lettuce drop pathogen, and that this beneficial insectary plant could increase soilborne inoculum of S. minor. Growers should therefore be aware of the host status of beneficial insectary and other plants interplanted with crops.
References: (1) P. Chitrampalam et al. Phytopathology 101:358, 2011. (2) K. V. Subbarao. Plant Dis. 82:1068, 1998.