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A. R. Chase*
University of Florida, IFAS,
Central Florida Research and Education Center - Apopka
CFREC-Apopka Research Report, RH-90-25
Research conducted during the past 10 years on disease control for ornamental plants has included screening new fungicides and bactericides as well as testing expansion of labels necessary for existing products. Several new fungicides have become available for testing on ornamental crops during the past two years. These chemicals were tested on some of the following diseases: Alternaria leaf spot of schefflera, Helminthosporium leaf spot of areca palm, Phytophthora stem rot of aphelandra, Rhizoctonia aerial blight of Boston fern, and Rhizoctonia stem rots of impatiens and vinca.
Alternaria Leaf Spot of Schefflera
Alternaria leaf spot of scheffleras (Brassaia actinophylla) is-caused by Alternaria panax which is also a pathogen of aralias and dwarf schefflera. Plants were sprayed with fungicides to the point of drip on a weekly interval between 27 September and 11 October. Treatments included triflumizole at two rates (Terraguard 50WP at 4 or 8 oz/100 gal), an experimental compound (G696 at 30 oz/100 gal), and iprodione (Chipco 26019 50WP at 24 oz/100 gal). Plants were inoculated with spores of the pathogen on 2 October and the number of spots per plant was determined on 12 October. Only Chipco gave a significant degree of disease control (Figure 1). G696 caused severe stunting and marginal chlorosis and necrosis.
Helminthosporium Leaf Spot of Areca Palm
Helminthosporium leaf spot is the most common disease of areca palm (Chrysalidocarpus lutescens) and is frequently caused by Exserohilum rostratum (= Helminthosporium rostratum). In the 1990 test, two rates of triflumizole (Terraguard 50WP, 4 and 8 oz/100 gal), one rate of mancozeb (Manzate 200 80WP, 1.5 lb/100 gal), and one rate of iprodione (Chipco 26019 50WP, 1 lb/100 gal) were tested. Fungicides were applied as a spray on a weekly interval from 28 June to 9 August. Plants were naturally infected and number of lesions per plant was recorded on 10 August. Best disease control was achieved with Manzate 200, Chipco 26019 or Terraguard at the lower rate (Figure 2). Control with the higher rate of Terraguard was not acceptable. It is not clear why a lower rate of this compound gave good disease control when a higher rate failed to give any disease control.
Phytophthora Stem Rot of Aphelandra
Phytophthora stem rot of many foliage plants is caused by Phytophthora parasitica. In 1990, a trial on zebra plant (Aphelandra squarrosa 'Dania') was performed with both foliar sprays and drenches of a number of fungicides. Sprays were applied to the point of drip and drenches were applied at the rate of 1 pint per ft2 on 4 September. Fungicide treatments applied as a foliar spray included: a combination of fosetyl aluminum and iprodione (Aliette 80WP at 2.5 lb and Chipco 26019 50WP at 1.0 lb/100 gal), Aliette 80WP alone at 2.5 or 5.0 lb/100 gal, and Aliette 80WG at 5.0 lb/100 gal. Fungicides applied as drenches were a combination of fosetyl aluminum and iprodione (Aliette 80WP at 0.4 lb and Chipco 26019 50WP at 0.5 lb/100 gal), Aliette 80WP alone at 0.4 or 0.8 lb/100 gal, Aliette 80WG at 0.8 lb/100 gal, metalaxyl (Subdue 2E at 2.0 oz/100 gal), and a combination product of benomyl and metalaxyl (Varsity 42WP at 1 lb/100 gal). Plants were inoculated with a mycelial slurry (added to the potting medium) on 10 September and symptoms were monitored every three days for about three weeks. Disease severity was rated from 1 (no disease) to 5 (dead). Symptoms of stem rot are given for 24 September (Figures 3,4). Best disease control was achieved with the foliar spray of Aliette 80WP at the high rate, the drench of Aliette 80WP at the high rate, and the spray and drench of Aliette 80WG. The combination of Chipco and Aliette as a drench and the Aliette 80WP as a drench at the low rate gave poorest disease control. Other fungicides gave intermediate degrees of control. Unfortunately, by the end of the trial none of the fungicides gave significant disease control (statistically), although all plants treated with the Aliette 80WG as a spray remained healthy.
Rhizoctonia Diseases of Boston Fern, Vinca and Impatiens
A series of trials was conducted on Rhizoctonia diseases of ornamentals since these diseases are not all caused by the same organism and different plants show different symptoms. Two types of trials on Rhizoctonia diseases are included in this report. The most common disease caused by Rhizoctonia on Boston fern (Nephrolepis exaltata) is called aerial blight. The second trial series employed two bedding plants, vinca (Catharanthus roseus and common impatiens (Impatiens Wallerana) which commonly suffer from Rhizoctonia stem rot. Plants were obtained from commercial producers, established in Vergro potting medium and grown under standard conditions. Ten single plant replicates were used for each of the treatments in each trial.
Control of aerial blight of Boston fern was attempted as a foliar spray with a variety of chemicals (Figure 5). Plants were sprayed weekly from 27 September to 11 October. Treatments included two rates of triflumizole (Terraguard 50WP at 4 and 8 oz/100 gal), one rate of an experimental compound (G696 at 30 oz/100 gal), and one rate of benomyl (Benlate 50DF at 16 oz/100 gal). Inoculation with an isolate of R. solani AG4 from fern occurred four days after the first fungicide treatment. The percent of the plant foliage with symptoms of aerial blight was recorded on 15 October. Excellent control was seen on plants sprayed with any fungicide (Figure 5). Both rates of Terraguard gave a high degree of control although G696 and Benlate 50DF gave a slightly higher degree of control. Ferns sprayed with G696 showed severe phytotoxicity as evidenced by stunting, marginal chlorosis and marginal necrosis.
Rhizoctonia stem rot trials were conducted with vinca and common impatiens. Fungicides were applied as drenches (about 1 pint/ft2) on 28 June (first trial) or 2 August (second trial). Inoculations with a mycelial slurry occurred 4 days after the fungicide application. Fungicides used in the first trial were two formulations of PCNB (pentachloronitrobenzene) (Terraclor 75WP at 4 oz/100 gal) and (Terraclor 50DF at 6 oz/ 100 gal) and triflumizole (Terraguard 50WP at 4 oz/100 gal). The percentage of the plant with stem rot was rated between 1 and 2 weeks after inoculation for impatiens and about 2 to 3 weeks after inoculation for vinca. Some disease control was achieved in the week following the inoculation but it rapidly collapsed as disease pressure increased for both impatiens and vinca (Figure 6). Since disease pressure may have been too high to properly evaluate control of Rhizoctonia stem rot on these bedding plants, a second test was performed.
The same treatment was used in the second trial with the following two new ones added: benomyl (Benlate 50DF at 16 oz/100 gal), and a combination product of benomyl and metalaxyl (Varsity 42WP at 16 oz/100 gal). In this trial, fungicides were applied on 2 August and plants were inoculated with a lower dosage of the pathogen on 6 August. As with the first trial disease control for the impatiens was initially good but generally failed to maintain control for even four weeks. Both Varsity 42WP and Benlate 50DF gave excellent disease control with the other fungicides giving slight control (Figure 7). With the exception of Terraguard, stem rot of vinca was more readily controlled with all fungicides, giving a significant degree of control (Figure 7). Apparently, none of these fungicides can control Rhizoctonia stem rot on these bedding plants when disease pressure is high. Prevention of this disease through use of clean plants, pots and potting media is the most important method of control.
Conclusions
1. Chipco 26019 50WP gives excellent control of Alternaria leaf spot of schefflera.
2. Manzate 200 80WP, Chipco 26019 and Terraguard 50WP (at 4-8 oz based on previous year's data) give excellent control of Exserohilum leaf spot of Arec Palm.
3. Phytophthora stem rot of zebra plant was best controlled by a foliar spray of a new formulation of Aliette (80WG).
4. Benlate 50DF and Terraguard 50WP gave excellent control, without phytotoxicity, of aerial blight on Boston fern.
5. Controlling Rhizoctonia stem rot on impatiens and vinca is dependent on disease pressure. Under conditions of very high pressure none of our fungicides can control this disease.
*Professor of Plant Pathology, Central Florida Research and
Education Center - Apopka, 2807 Binion Rd, Apopka, FL 32703-8504.
Figure 1. Effect of fungicides on severity of Alternaria leaf spot of schefflera (Brassaia actinophylla) caused by Alternaria panax.
Figure 2. Effect of fungicides on severity of Helminthosporium leaf spot of areca palm (Chlysalidocarpus lutescens) caused by Exserohilum rostratum.
Figure 3. Effect of fungicide sprays on severity of Phytophthora stem rot of zebra plant (Aphelandra squarrosa) caused by Phytophthora parasitica.
Figure 4. Effect of fungicide drenches on severity of Phytophthora stem rot of zebra plant (Aphelandra squarrosa) caused by Phytophthora parasitica.
Figure 5. Effect of fungicides on severity of Rhizoctonia aerial blight of Boston fern (Nephrolepis exaltata) caused by Rhizoctonia solani.
Figure 6. Effect of fungicides on severity of Rhizoctonia stem rot of common impatiens (Impatiens wallerana) and vinca (Catharanthus roseus) caused by Rhizoctonia solani AG4 - Test 1
Figure 7. Effect of fungicides on severity of Rhizoctonia stem rot of common impatiens (Impatiens Wallerana) and vinca (Catharanthus roseus) caused by Rhizoctonia solani. AG4 - Test 2.
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