The first article I wrote for GPN was on Botrytis prevention and control, and it appeared in October of 1998. Five years is probably long enough to wait to do an update since we have maintained an active research program in fungicide control of Botrytis on ornamentals trials each winter. During the past two years, we also started working on Sclerotinia blight on petunia.
Botrytis and Sclerotinia are related fungi, with Botrytis cinerea being the asexual state of Sclerotinia. We often see both fungi present in infected plants when the season changes from winter to spring. Botrytis blight is very common on dahlia, fuchsia, geranium, cyclamen, exacum, poinsettia, pansy and lisianthus. We have seen Sclerotinia crown rot or blight primarily on alyssum, petunia, lilies, lobelia, wallflower, pansy, stock, larkspur and many other bedding, perennial and cut flower crops. On some crops, Botrytis cinerea infections are confined to flower spots; while on others, cutting rot, stem rot and leaf spot also occur. Sclerotinia spp. usually causes blight (on bedding plants) and stem or crown rot (on perennials and cut flowers).
When I worked in Florida I rarely saw either of these diseases. Since returning to California 10 years ago, I have become much more familiar with these pathogens on our ornamentals. We have found chlorothalonil (Daconil, Syngenta Professional Products, and Spectro, Cleary Chemical), fenhexamid (Decree, SePRO), fludioxinil (Medallion, Syngenta Professional Products) and iprodione (Chipco 26019 and 26GT, Bayer Environmental Science) to be the most effective fungicides for prevention and eradication of Botrytis blight. These four active ingredients fall into distinct and separate chemical classes, making rotation for resistance management reasonably easy to accomplish. Since resistance to fungicides is one of the key concerns with Botrytis, rotation should be the cornerstone of every Botrytis control program.
Over the past two years we have looked at a variety of other products, including Agri-50 (a combination of several ingredients, Cal Agri Products), Endorse (polyoxin-D, Cleary Chemical), CX-7001 (an experimental product) and BAS500 (a new strobilurin, BASF Corporation). We have also looked at some older products in new combinations like Stature (mancozeb and dimethomorph, SePRO). There were multiple trials performed on cyclamen flowers (two trials performed), two performed on pansy, one on ranunculus, one on exacum and four on geranium in this time frame.
In some cases, we performed trials on plants that were infected with Botrytis blight (geranium especially), while in trials on other plants, we initiated treatments before Botrytis started. Products were usually applied every week for 4-6 weeks, and we rated disease every week. In addition, we always rated phytotoxicity and residue. Our greenhouses are only slightly under our control, with minimum temperatures fixed at 50º F. Fungicide applications were always made when temperatures exceeded 55º F but were below 85º F. Nearly all of our work on Botrytis is performed during the winter months since the fungus thrives on wet, cool conditions. Surprisingly, we performed the exacum stem rot trial last summer by inoculating wounded stems. This was a good demonstration of the high susceptibility of exacum to Botrytis and the wide range of conditions this pathogen can tolerate.
Figure 1, left, shows the results of the above described 10 trials for disease control as well as residue. Overall, the best fungicides remain unchallenged, with Chipco 26GT, Decree and Medallion giving the best control. There are a number of new products that showed the potential for very good control, including Agri-50, BAS-500, CX-7001 and Endorse. Of these, only Endorse is labeled at this time. The low rate (0.55 lb. per 100 gal.) of Endorse was not effective for Botrytis control in our trials.
In a few cases, phytotoxicity occurred resulting in loss of Botrytis control. This was especially apparent with chlorothalonil (Daconil Ultrex or Spectro) on cyclamen flowers and copper on many plants (previously reported trials as well as those here). We also saw this problem with some newer products like Agri-50, which was very effective when used at 0.16 percent but failed to control Botrytis when used at 0.33 percent or 0.5 percent.
The most severe residue was usually slight to moderate and occurred with Stature (due to mancozeb), Daconil Ultrex, Spectro and Endorse (on plants with hairy leaves like zonal geraniums). Most of the other products we tested had very slight to slight residue on these crops. One of the things making residue a problem is the drying pattern on some leaves. The spray tends to concentrate in spots before drying, resulting in typical spray residue patterns. Some of the product manufacturers recommend use of a wetting agent (like Capsil, The Scotts Company) to reduce residue, especially on poinsettia. The wetting agent is effective since it keeps the fungicide spread out evenly on the leaf surface, allowing it to dry in a less conspicuous layer. Be sure to check plant safety when adding a wetting agent to your Botrytis spray since not all labels allow additions of these products, and not all plants will tolerate these mixtures.
In 2002, we performed three trials for control of Sclerotinia blight on petunia but only gathered disease data in two of them (see Figure 2, page 31). One of the most obvious things we found was that petunias are really sensitive to nearly all of the fungicides we tried. The copper products resulted in little obvious phytotoxicity but no control of Sclerotinia. The strobilurins (BAS 500, Compass O, Cygnus and Heritage) gave different levels of control and different levels of phytotoxicity. BAS 500 was the safest, but Heritage was the most effective in this group. Chlorothalonil (Daconil Ultrex and Pathguard) were reasonably safe and gave very good to excellent control. Iprodione was one of the best active ingredients, giving excellent control, but each fungicide we tried (Chipco 26019, Chipco 26GT and Sextant) resulted in moderate to severe damage on petunias. For some reason, we failed to test Medallion but hope to do so in the upcoming months. Since petunias are so sensitive, we are planning to perform trials on at least one other crop as well.
Hopefully, it will not take another five years before we have something new to report on Botrytis and Sclerotinia control on ornamentals. In the meantime, make sure you rotate your products, avoid causing phytotoxicity by spraying when conditions promote drying and follow the labels.
Chase Research Gardens summarizes trials conducted over the past five years.