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A.R. Chase and T.A. Mellich*
University of Florida, IFAS,
Central Florida Research and Education Center - Apopka
CFREC-Apopka Research Report, RH-92-4
Fungicides have continued to play an important role in disease control strategies for many ornamental producers. New fungicides whose activity ranges are not currently known are becoming available for testing on ornamental crops. In addition, tests designed to help expand the labels of registered fungicides are an integral part of the CFREC-Apopka fungicide research program. The diseases caused by fungi which were researched during the past year were: Botrytis blight of geranium, Corynespora leaf spot of lipstick vine, Fusarium leaf spot of dracaena, and powdery mildew of gerber daisy.
Botrytis Blight of Geranium
Botrytis blight of geranium (Pelargoniun hortorum) is caused by Botrytis cinerea which is a pathogen of most flowering and foliage crops. Two tests were performed in 1991. In test 1, plants were sprayed with fungicides to the point of drip on a weekly interval between 18 July and 1 August 1991. Treatments included EXP10064B (a numbered compound from Rhone Poulenc) at two rates (2 and 4 oz per 100 gal), iprodione (Chipco 26019 50WDG) at two rates (16 and 32 oz per 100 gal), a mixture of EXP10064B and iprodione (2 and 16 oz per 100 gal, respectively), a mixture of EXP10221 (a numbered compound from Rhone Poulenc) and iprodione (2 and 16 oz per 100 gal, respectively), and chlorothalonil (Daconil 2787 75WP at 24 oz per 100 gal). Control plants were sprayed with water only. Plants were naturally infected with B. cinerea by 7 August at which time the number of active lesions per plant was determined. All of the treatments gave a good degree of control of Botrytis blight on geranium without any evidence of phytotoxicity (Fig. 1).
The second test was performed with the following treatments: noninoculated and inoculated controls sprayed with water, thiophanate methyl (Domain FL at 20 oz per 100 gal), chlorothalonil (Daconil 2787 90WDG at 20 oz per 100 gal), and ASC-66825 (a numbered compound from ISK Biotech) at two rates (4 and 16 oz per 100 gal). Treatments were applied by spraying to drip weekly from 16 January to 6 February 1992. Plants were inoculated with spores of B. cinerea on 31 January although, within 1 week, it became evident that all plants had been naturally infected with the pathogen. Thiophanate methyl gave moderate disease control while chlorothalonil and the two rates of ASC-66825 gave complete disease suppression as indicated by the number of leaves with active lesions on 10 February (Fig. 2). No evidence of phytotoxicity was noted.
*Professor of Plant Pathology and Biological Scientist, Central Florida Research and Education Center - Apopka, 2897 Binion Road, Apopka, FL 32703-8504.
Corynespora Leaf Spot of Lipstick Vine
Corynespora leaf spot is the most common fungal disease of lipstick vine (Aeschynanthus pulcher) in Florida. It is caused by Corynespora cassiicola which also causes leaf spot diseases on zebra plant (Aphelandra squarrosa) and variegated weeping fig (Ficus benjamina). Treatments in the first test included noninoculated and inoculated control plants sprayed with water, two rates of chlorothalonil (Daconil 2787 90WDG at 20 and 40 oz per 100 gal), two rates of ASC-66791 (a numbered compound from ISK Biotech at 32 and 64 oz per 100 gal), and a combination product containing thiophanate methyl and mancozeb (Zyban 75WP at 24 oz per 100 gal). Fungicides were applied as a spray to drip on a weekly interval from 15 August to 12 September 1991. Plants were inoculated with a spore suspension of the pathogen on 26 August and the number of lesions per plant was recorded on 16 September 1991. Excellent disease control was achieved with all of the fungicides tested (Fig. 3) and no signs of phytotoxicity developed during the test.
A second test was performed later in 1991 using mancozeb (Manzate 200 80WP at 24 oz per 100 gal), two rates of ASC-66825 (at 4 and 16 oz per 100 gal), and two rates of a quaternary ammonium product (Greenshield at 2 and 4 oz per 100 gal). Noninoculated and inoculated control plants were sprayed with water only. Treatments were applied weekly from 4 to 18 December 1991 except for Greenshield which was applied daily from 4 to 21 December. Plants were inoculated with a spore suspension on 6 December and rated for the number of lesions on 23 December. Manzate 200 gave little disease control in this test. Optimal control was achieved with weekly applications of ASC-66825 at the 4 oz rate or daily applications of the quaternary ammonium product at the 2 oz rate (Fig. 4). Daily applications of the higher rate of the quaternary ammonium product resulted in slightly increased levels of disease which may indicate a degree of phytotoxicity which provided more infection sites for the pathogen. The same reaction appeared to occur with the 16 oz rate of ASC-66825.
Fusarium Leaf Spot of Dracaena
Fusarium leaf spot of red-edge dracaena (D. marginata) is caused by F. moniliforme and occurs on many other species and cultivars of Dracaena. In 1991, a test was performed using weekly sprays of a combination product containing thiophanate methyl and mancozeb (Zyban 75WP at 24 oz per 100 gal), two rates of ASC-66825 (at 4 and 16 oz per 100 gal), and chlorothalonil (Daconil 2787 90WDG at 20 oz per 100 gal), and daily applications of three rates of quaternary ammonium product (Greenshield at 1, 2, and 4 oz per 100 gal). Noninoculated and inoculated control plants were included as well and were sprayed with water. Weekly sprays were applied on 5 and 12 December and daily sprays were applied from 3 to 16 December 1991. Plants were inoculated with a spore suspension added to the whorl of each plant on 6 December and the number of lesions per plant was recorded on 17 December. Best disease control was achieved with chlorothalonil or the combination of thiophanate methyl and mancozeb, although the higher rate of ASC-66825 and the lower rate of the quaternary ammonium product also gave very good disease control (Fig. 5). The increasing number of lesions on plants sprayed with the two higher rates of the quaternary ammonium appears to be due to development of a low level of phytotoxicity as noted on lipstick vine.
Powdery Mildew on Gerber Daisy
A test was performed during the winter of 1991 with gerber daisies (Gerbera jamesonii) naturally infected with powdery mildew (Oidium sp.). The following treatments were included: water sprayed control, two rates of chlorothalonil (Daconil 2787 90WDG at 20 or 40 oz per 100 gal), two rates of ASC-66825 (at 4 and 16 oz per 100 gal), and two rates of ASC-66900 (a numbered compound from ISK Biotech at 25 and 50 oz per 100 gal). Weekly sprays were applied from 18 December 1991 to 22 January 1992. The percentage of the leaf area covered with powdery mildew was estimated on 28 January, as well as plant height and the degree of foliar necrosis (phytotoxicity). Excellent control of powdery mildew was achieved with ASC66825 at both rates (Fig. 6). Neither chlorothalonil nor ASC-66900 gave a high degree of powdery mildew control in this test. Unfortunately, ASC-66825 resulted in both severe leaf burn and stunting, making its use on this gerber daisy impractical (Figs. 7 and 8).
Conclusions
The following conclusions are based on the tests reported in this paper and reflect the use of registered products whenever possible. The numbered compound ASC-66825 has proven to have a wide activity range and has been safe at 4 oz per 100 gal on the plants tested except for gerber daisy. Further work is planned for this compound.
1. Best control of Botrytis on geranium was achieved with weekly applications of Daconil, Chipco 26019, EXP 10064B or ASC-66825 with good control also given by Domain.
2. Excellent control of Corynespora leaf spot of lipstick vine was achieved with weekly
applications of Zyban, Daconil, ASC-66825 (4 oz per 100 gal), ASC-66791, or the daily applications of Greenshield (2 oz rate per 100 gal).
3. Best control of Fusarium leaf spot of dracaena was achieved with Daconil or Zyban, although the lower rate of Greenshield (1 oz per 100 gal) or ASC-66825 (16 oz per 100 gal) also gave very good disease control and did not cause phytotoxicity.
4. Excellent control of powdery mildew on gerber daisy was achieved with the numbered compound ASC-66825 although its applications resulted in severe leaf burn and stunting of the plant making its use on gerber daisy impractical.
Figure 1. Effect of fungicides for control of Botrytis
blight of geranium (Test 1).
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Figure 2. Effect of fungicides for control of Botrytis blight of geranium (Test 2).
Figure 3. Effect of fungicides on control of Corynespora leaf spot of lipstick vine.
Figure 4. Effect of fungicides and a quaternary ammonium product on control of Corynespora leaf spot of lipstick vine.
Figure 5. Effect of fungicides and a quaternary ammonium product on control of Fusarium leaf spot of dracaena.
Figure 6. Effect of fungicides on control of powdery mildew on gerber daisy.
Figure 7. Effect of fungicides on height of gerber daisy.
Figure 8. Effect of fungicides on phytotoxicity on gerber daisy.
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1 - no phytotoxicity and 5 - dead plant
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