[Image: Stella de Oro]
Daylily is an economically important nursery crop in the southeastern U.S. and was relatively disease-free before the arrival of daylily rust, caused by Puccinia hemerocallidis, in 2000. Although rust-resistant varieties have been identified and susceptible varieties are being phased out of production, commercial growers rely solely on fungicides to control this rust, which has increased the production cost of this crop. Several fungicides are effective against rust, including azoxystrobin (Heritage), propiconazole (Banner Maxx) and chlorothalonil (Daconil Ultrex). However, there is concern about rust resistance to these products because of their repeated use.
Biofungicides are biological or bio-rational alternatives that can reduce disease by using different modes of action. If effective, biofungicides can provide new resistance-management tools, and they are usually less expensive and more environmentally friendly than conventional fungicides.
Materials and Methods: We tested K-Phite (phosphoric acid, Plant Food System), Vital (phosphoric acid, Luxembourg-Pamol), Actinovate (Streptomyces lydicus, Natural Industries), Rhapsody (Bacillus subtilis, AgraQuest), MilStop (potassium bicarbonate, BioWorks) and Citrex (organic acids, Special Nutrients Inc.) in our trial in rotation or as tank-mix partners for their preventative or early-stage curative effects on daylily rust.
A total of 220 pots of Stella de Oro were used in two trials. We selected this variety because of its popularity as the first publicly recognized daylily and its wide use in the landscape. Although it was reported as a moderately resistant variety, we had observed two outbreaks of rust on this variety for two consecutive years prior to this study (April and September). We did not use highly susceptible varieties because they are very unlikely to be in production in the future.
In each trial, the experimental design was a randomized complete block design with 4 blocks and 5 pots in each block as subsamples. The subsamples were placed by surrounding a center pot with four pots such that their leaves were touching the leaves of the plants in center pot. In Trial 1, plants were not cut back at the initiation of treatment. In Trial 2, foliage of the plants in 4 surrounding pots was cut back to 1 inch above the top of the media, and the foliage of the plant in center pot was uncut and infested with rust.
Rust was evaluated using a subjective visual rating system with a scale of 0 to 5, where 0 indicates no rust infection, 1 = observable rust infection on lower leafs (about 20% foliage infected based on number of leaves showing any yellow spots), 2 = about 40% of the foliage showing yellow spots, 3 = about 60% of the leaves showing infection with orange rust spores present of the lower leaves, 4 = most of the foliage showing yellow spots with lower half of foliage showing orange rust spores and dry leaves and 5 = plants covered with orange spots.
An overall quality rating was also assigned to each plant and correlated to rus[Image: Fig. 1]t ratings. Based on this correlation, rust ratings greater than 1 represent that plants start to lose aesthetic quality (data not shown). Ratings were conducted 4, 5 and 6 weeks after treatment began.
The first treatments were applied on April 10 and September 15, 2006, for Trials 1 and 2, respectively. Plants were naturally infested with rust at the time of first application with a rust rating at about 1. Foliar applications were made with a hand-held sprayer. Treatment solutions were sprayed to runoff except Milstop, which was sprayed so as just to wet the foliage. All rotation or tank-mix treatments were applied every 10 days.
A total of 11 treatments were applied in Trials 1 and 2. These are: K-Phite (4 pt/100 gal), Actinovate (12 oz/100 gal) and MilStop (2 lb/100 gal) applied alone or at the above rates in rotation with Banner Maxx (4 oz/100 gal). K-Phite was also applied at the above rate with 1ml/gal CapSil to see if adding an adjuvant would increase its efficacy. CapSil alone, Banner Maxx alone and Banner Maxx in rotation with water were used as controls in addition to the untreated control (water).
For each trial, the analysis of variance (ANOVA) was performed by using PROC GLM (SAS Institute Inc. Cary, NC), and means were compared with the appropriate Fisher’s protected LSD at a = 0.05.
Results: A rust rating equal to or less than 1 was considered aesthetically acceptable for marketing. In Trial 1, where foliage were not cut back, at 4 WAT, plants treated with Actinovate alone, K-Phite in rotation with Banner Maxx and Banner Maxx alone or in rotation with water had less severe rust than the untreated control (Fig. 1). However, none of these treatments resulted in aesthetically acceptable rust control. When these same treatments were applied to plants that had been cut back for new growth in Trial 2, K-phite was the only material t[Image: Fig. 2]hat contributed to its rotation program and was as effective as Banner Maxx alone (Fig. 2). Actinovate and MilStop did not contribute to the rotation program (Fig. 2).
Overall, biofungicides evaluated in this study were effective compared to untreated control under high rust pressure when applied to plants that had been cut back for new growth. However, they were less effective than the current standard except that K-Phite showed promise as a rotation partner. More trials are needed with K-Phite and phosphoric products.
Economically, if consistent effects can be obtained, adding K-Phite into a rotation program can reduce overall cost and possibly delay resistance development. Results from this study also suggest that more effective control can be obtained when fungicides are used in combination with cutting back plant foliage for new growth after rust symptoms are detected, which provides a preventative control rather than curative.