Spineless selections of Carthamus tinctorius (safflower) are grown as commercial field grown cutflower crops in coastal California. In 2010, field plantings of spineless safflower in Santa Clara County developed symptoms of a wilt disease. Affected plants were stunted and slow to develop. As plants developed flower buds, lower leaves turned yellow and wilted. As disease developed, lower leaves turned tan and desiccated; in extreme cases, the entire plant wilted and died. Examination of the taproot and stem vascular tissue revealed a tan to light brown, longitudinal, vascular discoloration. Disease distribution was patchy but in some plantings up to 50% of the plants were unharvestable due to loss of quality or plant death. Isolation from symptomatic vascular tissue consistently resulted in the recovery of a fungus with white aerial mycelium, verticillate conidiophores, single-celled, ovoid to ellipsoid, hyaline conidia, and solitary black microsclerotia that were rounded to elongated or irregular in shape. Three single conidial isolates were chosen for species and race identification after DNA extraction using the FastDNASPIN Kit (MP Biomedicals, Solon, OH). Verticillium species-specific PCR amplified a 500-bp amplicon that is specific to Verticillium dahliae from all three isolates. All three isolates also amplified the race 2-specific 270-bp band in PCR. No amplification was observed in race 1-specific PCR. Based on morphological and molecular data, the fungus was identified as V. dahliae (1,3). Pathogenicity of two isolates was tested individually by soil drench inoculations using 10 ml of conidial suspensions (7 × 106 conidia/ml) for each of 10 containerized plants grown in a peat moss mix in 7.6 cm diameter pots. Five safflower selections were inoculated and maintained in a greenhouse. After 6 weeks, as plants began to form flowers, inoculated plants showed lower leaf dieback and plant wilting. Vascular discoloration was observed when plants were dissected. V. dahliae was consistently recovered from symptomatic tissue. Control plants that only received water did not develop symptoms. The experiment was repeated and the results were consistent. To our knowledge, this is the first report of Verticillium wilt of ornamental, spineless safflower. This disease has been reported previously on agronomic safflower grown as an oilseed crop (2). This finding has significance for coastal crop rotation decisions as ornamental safflower is yet another host that could augment V. dahliae soil inoculum levels for crops such as strawberry and vegetables.
References: (1) P. Inderbitzin et al. PLoS One 6: e28341, 2011. (2) J. M. Klisiewicz. Plant Dis. 65:237, 1981. (3) Maruthachalam et al. Phytopathology 100:1222, 2010.