Yan Chen, Ferrin, Donald M., Bracy, Regina P. | 9/12/2007 2:52:27 AM
Daylily is an important nursery crop and was relatively disease-free before the arrival of daylily rust, caused by Puccinia hemerocallidis in 2000. Rust-tolerant varieties are being identified and susceptible varieties are being phased out of production. Several fungicides are effective against rust, including azoxystrobin (Heritage), propiconazole (Banner Maxx) and chlorothalonil (Daconil Ultrex). Relying on fungicides to minimize rust symptom increases production cost and concerns on potential resistance of rust to these products.
Biofungicides are biological or bio-rational alternatives that can reduce symptom by using mode of actions different from systemic fungicides. If effective, biofungicides can be used as resistance-management tools, and they are usually less expensive.
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 rotation or as tank-mix partners for their preventative or early-stage curative effects on daylily rust.
Stella de Oro, a popular landscape variety, was used in two trials conducted from 2005 to 2006. This variety is moderately tolerant to rust infestation. At the Hammond Research Station, we had observed two outbreaks per year of rust symptom, in April and September on this variety for two consecutive years prior to this study. 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 replications (clusters). Each cluster had 5 pots placed close to each other. The center pot had one 2-year old plants showing rust symptom. The four surrounding pots were newly planted and was either cut back to 2 inch tall (Trial 1) or left un-pruned (Trial 2).
Trial 1 was conducted from April 10 to June 5 2005. Plants were naturally infested and showing early rust symptoms (rated for an average 1 = minor symptom, according to a 1 to 5 rating scale). A total of 11 treatments were applied: 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). 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 for a total of 6 applications.
Trial 2 was conducted from September 15 to October 30, 2006 with the same treatments. The center plants in each cluster showed rust symptom for an average rating of 1. Leaves on the four surrounding test plants were cut back to 2 inch, and new leaves growing from these plants were sprayed with the products.
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 assigned to each plant and correlated to rust ratings. Based on this correlation, rust ratings greater than 1 represent that plants start to lose aesthetic quality (data not shown). Rust ratings equal to or less than 1 was considered still aesthetically acceptable. Ratings were assessed at 4, 5 and 6 weeks after treatment began.
Results: In Trial 1, where foliage were not cut back and new plants were placed next to infested center plant, at 4 WAT, plants treated with Actinovate alone, Banner Maxx alone, Banner Maxx in rotation with water, or its rotation with K-Phite had less severe rust than the untreated control (Fig. 1). However, none of these treatments resulted in aesthetically acceptable plants due to moderate to severe rust symptom. The role of K-Phite in a rotation program is still unclear because its effect was less significant than rotation program with water.
In Trial 2, the same treatments were applied to plants that had been cut back for new growth. Plants treated with K-Phite, K-Phite plus CapSil, and MilStop had less severe rust ratings than the water or CapSil control (Fig. 2). Actinovate plus CapSil had similar rust rating as CapSil control. However, all these treatments resulted in rust ratings more than 2.3, therefore, when used alone, these biopesticides will not provide sufficient control to grow quality daylilies. Among rotation treatments, Banner Maxx in rotation with Actinovate or MilStop resulted in similar ratings as its rotation with water, thus did not contribute to the rotation. K-phite was the only material that contributed to its rotation program resulting rust ratings as effective as Banner Maxx alone (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 Banner Maxx 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.