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Rhizopus head rot has been an important disease of cultivated sunflower (Helianthus annuus L.) in Texas. The disease has been also reported from California, USA 41 and other countries1,2,3). Rhizopus arrhizus Fischer1,2,6), R. oryzae Went & Prinsen-Geerligs4) and R. stolonifer (Ehrenb. ex Fr.) Vui112,6) have been reported to cause head rot of cultivated sunflower. No sunflower cultivars are resistant to Rhizopus spp. Attempts have been made to find sunflower for resistance to Rhizopus spp. in the greenhouse. For reliable greenhouse results, the inoculation technique must cause severe rot of all susceptible types of sunflower. We have compared the efficacy of several different inoculation methods1,4,5,6) for causing head rot in 4 cultivars, 2 hybrids, and 4 inbred lines of sunflower grown in greenhouse. This note describes the most effective technique for inoculating greenhouse grown sunflower plants with three species of Rhizopus. Rhizopus arrhizus and R. stolonifer were isolated from diseased sunflower heads in Texas and R. oryzae was supplied by J. M. Klisiewicz. The three pathogens were grown on an autoclaved wheat-grain medium. Wheat grains. 20g in 300ml of water, were soaked for overnight, and the grains were washed at least twice with tap water. After adding 60 ml tap water, the flask with wheat grains was autoclaved for 2hr at 121 C on two consecutive days. The sterile wheat-grain medium was seeded with two blocks of agar with mycelium (from 18-hr cultures on potato dextrose agar) and incubated at 20 to 25 C for 4 to 10 days. The medium was removed from the flask and cut into small pieces with a sterile scalpel. Heads at bud (yellow ray flower not visible), open-bud (ray flower visible but not opened), and full-bloom stages were inoculated with about 0.2g of the wheat-grain culture inserted with forceps to a depth of approximately 5mm into the receptacle

An effective technique for inoculating greenhouse-grown sunflower with three species of Rhizopus.