Phlox paniculata 'Blue Boy' and Rudbeckia hirta 'Indian Summer': Cultural Guidelines for Greenhouse Growth and Powdery Mildew Control

Little information is available about greenhouse production requirements of Phlox paniculata 'Blue Boy' and Rudbeckia hirta 'Indian Summer', both of which are extremely susceptible to powdery mildew. Some plant growth regulators (PGRs) have been reported to reduce severity of certain plant diseases. The objectives of these experiments were to: 1) define optimal fertilizer, irrigation rates, and media types for these cultivars; 2) determine optimal PGR rates for size control; and 3) determine effects of PGRs on powdery mildew severity on inoculated plants grown under optimal greenhouse conditons.

When looking at the variables height, average width, quality rating, and shoot dry weight, Phlox paniculata 'Blue Boy' grew best at 200 mg·L⁻¹ N in Fafard 3B medium and was not responsive to irrigation rate. Also, when looking at the same variables mentioned above, Rudbeckia hirta 'Indian Summer' grew best at 300 mg·L⁻¹ N in Scott's Sierra Perennial Mix at a high irrigation rate. The PGRs chlormequat chloride, daminozide/chlormequat chloride, and paclobutrazol were effective in controlling size of Phlox. During the first Rudbeckia experiment, paclobutrazol and uniconazole were effective in controlling plant size; in the second experiment daminozide, uniconazole and flurprimidol were effective.

In the fall experiment, 160 mg·L⁻¹ paclobutrazol was effective in reducing disease severity in Phlox; in the spring experiment, 4000 mg·L⁻¹ chlormequat chloride and 60 mg·L⁻¹ uniconazole were effective in reducing disease severity. In the Rudbeckia fall experiment, 160 mg·L⁻¹ of two forms of paclobutrazol were effective in reducing disease severity; in the spring experiment, only one of those forms (Bonzi) was effective.