Great Lakes Fruit, Vegetable & Farm Market EXPO Michigan Greenhouse Growers EXPO December 6-8, 2011 DeVos Place Convention Center, Grand Rapids, MI How To Choose What Works for Disease Control Where: Grand Gallery (main level) Room D Sponsor: OHP, Inc. Recertification credits: 1 (COMM CORE, PRIV CORE) Moderator: Ken Kuhajda, OHP,Inc, Medina, OH 2:00 pm How To Choose What Works for Disease Control: What a grower should understand about plant pathogens and what to do when they plague your plants. Dr. Hausbeck will discuss the most common pathogens and how to recognize the problem, how to get it diagnosed and measures to take to prevent and control the pathogen. Mary Hausbeck, Plant Pathology Dept., MSU 2:50 pm Session Ends
How to Choose What Works for Disease Control Dr. M.K. Hausbeck (517-355-4534) and Blair R. Harlan Michigan State University, Department of Plant Pathology, East Lansing, MI Botrytis Blight. The fungus Botrytis cinerea causes disease on greenhouse crops including leaf spots, blighting, stem cankers, and damping-off of young seedlings. Blighting is most common and affects stems, leaves, petioles, and blossoms. Botrytis produces large masses of gray conidia or spores (hence the name gray mold ) that are carried on air currents to healthy plants where blight can become established. On bedding and stock plants, Botrytis typically becomes established and produces conidia (spores) on older lower leaves that are near the moist soil surface and under the plant canopy. Botrytis also readily infects the broken or cut stem surface of stock plants and progresses downward, causing a dieback of the entire stem. When the weather is moist and humid, susceptible plants may need to be protected from Botrytis infection. Finding the beginnings of brown/gray fuzziness on lower leaves can signal the need for disease control measures. While fungicides alone cannot control Botrytis blight, they are often necessary partners in a successful management program that includes environmental control and sanitation. Even the best fungicides will not provide the needed control if the relative humidity is high (exceeding 85%) and the leaves stay wet for 6 hours or more in a 24-hour period. Plants may also be susceptible if they become wet from water dripping from overhead, dew, or condensation. Water allows the Botrytis conidia to germinate and penetrate the plant. Watering in the morning so that the foliage can dry rapidly is one way to minimize Botrytis. Reducing relative humidity by spacing plants further apart and providing good air circulation can be helpful. Reduce the relative humidity for a minimum of 24 hours immediately following the harvesting of cuttings to help dry the wounded stems and thereby limit stem blight. Geraniums are good test plants because they seem to be a Botrytis magnet. Fungicides are applied to the plants and allowed to dry prior to introducing Botrytis conidia. Each time a test is conducted, fungicides that are considered to be especially effective are included for a comparison. Over many years of testing fungicides, I consider Decree and fungicides containing chlorothalonil (Daconil is an example) to be standards for Botrytis because they consistently provide effective control. Botrytis A Team Botrytis B Team Daconil Weather Stik SC chlorothalonil Compass O WDG trifloxystrobin Decree 50DF fenhexamid Dithane DF mancozeb Chipco 26019 WDG, 26GT F iprodione Heritage WDG azoxystrobin Pageant 38WG pyraclostrobin + boscalid Insignia WG pyraclostrobin Veranda O WDG polyoxin D zinc salt Palladium WDG cyprodinil + fludioxonil Downy Mildew. Downy mildew occurs sporadically on some crops, but can be a predictable pest on others. There are many different pathogens that cause the downy mildew diseases and most are restricted to only one or a few plant crops. The downy mildew pathogen that infects rose is specialized and does not cause disease on other ornamentals, so if you ve been struggling with downy mildew on coleus, snapdragon or impatiens, the rose downy mildew pathogen is not the culprit! Downy mildew can occur on all aboveground plant parts, blighting the leaves and stems. Sometimes the first symptoms of downy mildew are confused with a nutrient deficiency or spray injury. On the leaves, spots may be purplish or brown and appear square since they may be limited by the larger veins. Downy mildew that
infects some plants, such as roses, doesn t always produce a fuzzy mat on the underside of the leaf that is noticeable without magnification. On other species, such as impatiens and coleus, sporulation may be observed easily on the underside of an infected leaf. When the infection is severe enough, leaves may drop from the plant, leaving the plant nearly devoid of foliage. Once the diseased leaves start dropping, the downy mildew is advanced and stopping it becomes very difficult. Since the downy mildew pathogen can lay quiet in tissue without noticeable blighting, it is possible to receive plants that appear healthy only to have symptoms develop later. It is also possible for the downy mildew pathogen to persist in a greenhouse or production facility, causing disease from one season to the next. A specialized spore (oospore) can remain dormant in infected plant debris and soil, surviving harsh weather conditions and allowing the downy mildew pathogen to survive between crops. Downy mildew is very responsive to weather cues; when the weather is favorable, downy mildew symptoms can explode almost overnight. Wet weather, high relative humidity, and overcast conditions are triggers to downy mildew disease. In outdoor growing facilities fog provides nearly the perfect weather for an outbreak. During wet weather, a fuzzy mat of fungal threads can coat the underside of the leaf. This is where the downy mildew pathogen reproduces via a spore type called sporangia. They develop and ripen during the night as long as there is darkness and at least 6 hours of continuous moisture. When the environment begins to dry in the early to mid-morning hours, the air currents or splashing water pluck the sporangia from their spore stalks and carry them to nearby healthy foliage. Downy mildew is most favored at temperatures around 60-65ºF. Temperatures that are too warm (80ºF and above) or too cold (40ºF and below) stop the disease. If the weather becomes hot and dry, the downy mildew pathogen will be halted at least for awhile, but it is possible for it to lay quiet in infected tissue and wait for cooler weather. MSU research tested the effect of temperature on the ability of Peronospora sp. to infect coleus at different temperatures. Infection was greatly reduced when temperature reached 77ºF and did not occur at 86ºF infection. A similar study was conducted to determine temperature effect on sporulation. Again, when temperatures reached 77-86ºF sporulation was significantly reduced. Coleus downy mildew epidemics have recently become a problem. Although it seems that all coleus cultivars may be affected by downy mildew, the amount of blighting and leaf dropping that results may vary. Cultivars may also differ in how many sporangia are produced on coleus leaves. This is an important characteristic because the sporangia spread the downy mildew disease. Research at MSU and Cornell University tested many coleus cultivars for resistance to downy mildew and have found these cultivars to be the least susceptible: Beauty, Black Ducksfoot, Dark Chocolate, Etna, Fairway Lemon, Fairway Orange, Fairway Red Velvet, Fairway Rose, Fairway Salmon Rose, Florida Sun Lava, Florida Sun Rose, Freckles, Fright Night, Giant Palisandra, Glory of Luxemborg, Gold Edge, Harlequin, Kiwi Fern, Midway Curly Magenta, Night and Glow, Pegasus, Russet, Saturn, Smoldering, Solar Furnace, Tapestry, Trailing Garnet Rose, Versa Lime, and Wild Streak. Fungicides may be needed to protect crops from downy mildew. MSU studies tested currently registered and unregistered products. One study included a new active ingredient called mandipropamid and compared it with registered products Heritage, Stature, and Subdue MAXX EC (applied as a drench) alone and in combination. In this trial, all treatments were effective and limited downy mildew. In another trial, fungicides tested included Stature DM 50WP, Pentathlon LF, Subdue MAXX EC (applied as a drench), and Insignia. A third study compared the new active ingredient, fluopicolide, and other registered products including FenStop SC, Stature DM 50WP, Pentathlon LF, and Segway 400SC. All of these treatments were very effective in preventing downy mildew. Applying fungicides for downy mildew must include a plan that delays the ability of downy mildew to become accustomed to a particular fungicide or active ingredient. This can be accomplished by mixing two fungicides together, but each one must have a different mode of action. Another common approach is to use a fungicide program that uses a different product or active ingredient for each spray.
Downy Mildew A Team Heritage WDG azoxystrobin Adorn SC fluopicolide Stature WP dimethomorph Dithane DF mancozeb Subdue MAXX EC mefenoxam FenStop SC fenamidone Powdery Mildew. Powdery mildew epidemics can occur without warning. Depending on how the disease is managed, there may be minimal impact or complete crop loss. The white talcum-like growths (colonies) start small but can rapidly blight the leaves, stems, and flowers of susceptible crops. Some powdery mildews are caused by Erysiphe cichoracearum which can infect many different annual and perennial flowers and vegetables. Powdery mildew can occur on all above-ground plant parts and when the fungus reproduces, the abundant conidia (spores) give a white, powdery or fluffy appearance. Severe infection can cause yellowing and withering of leaves and stunt plant growth. Powdery mildew can infect plants even when the relative humidity is low, but high relative humidity can prompt epidemics. Sometimes the powdery mildew progresses unnoticed until many plants are infected. Since the powdery mildews are a yearly problem, a long-term strategy is needed. Growing susceptible crops can be a challenge, and fungicides have typically played a key role. Powdery mildews are tricky and have been known to genetically adapt to overcome some of the most effective fungicides. Some plant species such as gerbera daisy and verbena are very susceptible and should be sprayed more frequently with the most effective fungicides. Other plant species may not need frequent applications but should be scouted regularly for signs of the disease. Powdery Mildew A Team Powdery Mildew A-/B Team Eagle EW/WP myclobutanil Compass O WDG trifloxystrobin Terraguard 50W triflumizole Heritage WDG azoxystrobin Insignia WG pyraclostrobin Pageant WG pyraclostrobin + boscalid Strike WDG triadimefon Zyban WSP t-methyl + mancozeb Pythium Root Rot. Pythium crown and root rot is a troubling disease for many growers and at times may seem unavoidable and tough to control. Pythium is a water mold and nibbles the feeding roots of plants, resulting in stunted growth and death. Pythium root rot is favored by growing conditions that are too wet, such as when media does not drain quickly or when weather doesn t allow rapid drying. Pythium can be introduced into a greenhouse via plant plugs or other prefinished plant material. This pathogen can also be a greenhouse resident that hibernates on dirty plant containers, benches, hoses, and greenhouse walkways, ready to become activated by the right crop and weather conditions. Although Pythium can be a problem on many annuals and perennials, it seems to favor certain crops (for example, geraniums and poinsettias) more than others. Sanitation is especially important in limiting root rot. First, use a pressure washer with soap and water when cleaning walkways, benches, etc. Second, treat cleaned surfaces with a disinfestant to remove any remaining unseen problems such as Pythium. Conditions that favor good plant growth and minimize stress make the plant less vulnerable to attack by a root rot. But if you ve done everything right and still find yourself with a Pythium problem, choosing the right fungicide tools can minimize your losses. Pythium root rot must be detected and identified early while fungicide drenches can be most effective. If Pythium has a significant head start, the root system of some plants will be too rotted and the fungicides will not be able to rescue them. If timely fungicide drenches with appropriate rates have been made and root rot continues to be a problem, it is time to take a hard look at the products used. If Subdue MAXX has been the only or primary fungicide used over the years, it is possible that the Pythium has become resistant and is no longer affected by this fungicide. To know for sure, the Pythium present in
your greenhouse can be tested. Some diagnostic clinics offer this service. To avoid the development of resistance, rotate among the different active ingredients available among fungicide products. Generally speaking, fungicides that control Pythium disease do not control Rhizoctonia and Thielaviopsis root rots. A fungicide such as Banrot 40WP is a mixture of two different active ingredients and targets all three root rots. Banrot can be a good choice when you need to make a treatment quickly and don t have time to determine which root rot pathogen is the culprit. If Pythium is diagnosed as the problem, fungicides that are specific for Pythium can be used and include Subdue MAXX (also available as Mefenoxam 2), Banol, Truban, Terrazole, and Aliette. Truban and Terrazole are often used by growers when their particular Pythium is resistant to Subdue MAXX. Other growers successfully rotate either Truban or Terrazole with Subdue MAXX in a program to control root rot and delay potential problems with fungicide resistance. Since Truban and Terrazole have the same active ingredient, rotating between these two fungicides is not recommended. Aliette is a tool for Pythium control that can be helpful if used early and if the disease is not severe. For the best control, the time between fungicide applications should not be stretched beyond the minimum interval listed on the label. Pythium A Team Pythium B/B- Team Truban WP/Terrazole WP etridiazole Adorn SC fluopicolide Banol EC propamocarb Aliette WDG fosetyl-al Subdue MAXX EC mefenoxam Captan WDG captan Heritage WDG azoxystrobin Fenstop SC fenamidone Segway SC cyazofamid Phytophthora Root Rot. Phytophthora is a cousin water mold to Pythium and can be a particularly devastating and difficult-to-control problem. Two types of Phytophthora (Phytophthora nicotianae and Phytophthora drechsleri) are usually found in floriculture crops and can cause root, crown, and foliar blights. Symptoms include brown-black cankers at the soil line and diseased roots. Phytophthora nicotianae can infect snapdragon, fuchsia, verbena, bacopa, vinca, African violet, and dusty miller to name a few. Phytophthora drechsleri may infect poinsettias. Losses can be especially severe where ample water and warm temperatures favor disease epidemics. Phytophthora (and Pythium) is difficult to control because it may produce different spore types that cause disease. Thick-walled oospores survive between crops on plant containers, benches, floors, and in potting media or soil. Other spores include lemon-shaped sporangia, thick-walled chlamydospores, and swimming zoospores. Under warm, wet conditions many sporangia develop on infected plants and lead to a rapid build-up and spread of disease in a short period of time. Sporangia form on the surface of roots, crowns, and other plant tissues, and may be dislodged from infected plants by irrigation or drainage water. Sporangia may directly germinate and cause infection. In water, sporangia release many smaller swimming spores called zoospores which can spread with moving water. Zoospores can survive and cause infection for 8 to 20 hours. They are attracted by root exudates from host crops. One zoospore is all that is needed to infect a plant. The cycle of plant infection and zoospore production can occur many times within a single growing season. Thus, low levels of infection early in the season may result in a rapid epidemic, if the disease is not immediately controlled. Once an epidemic has developed in a production site it is often difficult to determine how the Phytophthora was introduced, how it is spreading, and if it is surviving from year to year. Using genetic tools, MSU research showed that Phytophthora spread within the greenhouse can occur from zoospores that spread through the water. Research also showed how Phytophthora may spread among producers. For instance, snapdragon producers at two locations purchased plugs from the same supplier. The Phytophthora from the two locations was identical. While it is possible that the disease was introduced to the locations via infected purchased plugs, it is also possible that the Phytophthora was already
established at these production sites. The Phytophthora recovered from snapdragons was identical to the Phytophthora collected from the same facility in a previous year indicating the potential for this pathogen to survive even with a fallow period and treatment with a fumigant (methyl bromide/chloropicrin). In another example, verbena propagated at one greenhouse via cuttings was sold to two other greenhouses for finishing. The finding that the Phytophthora from all three locations were genetically identical suggests that infected plants could have spread P. nicotianae from the propagator to the two growers. Controlling the spread of Phytophthora spp. within and among production facilities can be difficult and there are two major challenges. First, Phytophthora must be kept out of the production site. This is particularly difficult with floriculture crops because of the widespread distribution of prefinished plants. Also, plants may not exhibit obvious symptoms until the infection is well established or the plant becomes stressed (e.g., over- or underwatering). Infected plants treated with fungicides may appear healthy until the fungicides wear off and Phytophthora increases. The second challenge is eradicating Phytophthora once it has been introduced. Removing visibly diseased plants will not prevent spore production and spread from plants showing few if any symptoms. Sanitation can limit disease and includes removing plant debris and disinfesting production surfaces. Routinely treating plants with fungicides including the standard Subdue MAXX can be helpful. However, Phytophthora can develop resistance to these fungicides and new management strategies and tools are needed. New Phytophthora products have been introduced with more on the horizon. MSU has tested these products for their ability to halt Phytophthora disease on several crops. Some products performed very well, keeping plants alive and healthy while inoculated plants that were not treated became severely diseased and died. Adorn has proved effective at controlling disease on pansy, poinsettia and snapdragon, FenStop and Insignia protected pansy and snapdragon, and Alude kept pansy plants alive and healthy. However, some products that have performed well on one crop but failed on another, so care must be taken to use a fungicide program that effectively rotates products to maintain disease control and minimizes the chance of fungicide resistance developing in Phytophthora. Phytophthora A Team Phytophthora B Team Adorn SC fluopicolide Aliette WDG fosetyl-al Alude L phosphorous acid salts Captan WDG captan FenStop SC fenamidone Insignia WG pyraclostrobin Stature DM WP dimethomorph Segway SC cyazofamid Subdue MAXX EC mefenoxam Terrazole WP etridiazole Rhizoctonia Root Rot. The Rhizoctonia solani fungus typically causes a dull brown to dark brown rot on cuttings or lower plant stems. Terraclor, Terraguard and Cleary s 3336/OHP 6672 applied as a drench have been an important tool in preventing Rhizoctonia and halting its spread. The fungicide, Banrot, contains a combination of fungicides (etridiazole + thiophanate-methyl) and provides broad spectrum control against both Pythium and Rhizoctonia. Biocontrol agents are becoming more widely available for use in controlling damping-off fungi such as Rhizoctonia. Information on how to use these products most effectively is still being gathered. In some instances, university trials have been conducted and may be helpful to growers in deciding when and how to incorporate these products into their damping-off management program. Growing mixes that are naturally suppressive to damping-off soilborne fungi are also available to growers. A suppressive medium is one that represses, restrains, or checks the growth of damping-off fungi. Intitially, interested growers may want to test the usefulness of suppressive media in their growing system by planting a small portion of their crop in suppressive media. Communication with sales representatives and extension personnel may be helpful to evaluate and perhaps modify growing systems using suppressive medium to achieve the best results.
Rhizoctonia A Team Rhizoctonia B+ Team Medallion WG fludioxonil Cleary s 3336 WP/OHP 6672 FL t-methyl Terraclor WP PCNB Captan WDG captan Terraguard 50W triflumizole Heritage WDG azoxystrobin Veranda O WDG polyoxin D zinc salt Black Root Rot. While root rots caused by Pythium and Rhizoctonia are the most common among greenhouse crops, black root rot is a serious threat to pansies, petunias, and vinca. Black root rot is caused by the fungus, Thielaviopsis basicola, and may also infect cyclamen, calibrachoa, poinsettia, primula, impatiens, snapdragon, verbena, phlox, begonia, and nicotiana. Plants with black root rot often show symptoms that mimic nutrient deficiencies, such as stunting and shriveling of older leaves. Leaves may turn yellow and the youngest leaves become stunted and tinged with red. In mild infections, older leaves are yellow-green with the veins retaining their green color. Black root rot may also affect the lower stem on crops such as poinsettia, causing cracks that appear black. Sanitation is the best preventive measure against black root rot. Once this fungus is established in a crop or greenhouse, an effective fungicide program is needed. Based on MSU studies, fungicides that have thiophanate-methyl as the primary active ingredient (Cleary s 3336 F is an example) should be used frequently at the high labeled rate. Good rotational products include Terraguard and Medallion since they have a different mode of action and were shown to be effective in MSU studies against black root rot. Choosing an effective fungicide to control black root rot is critical because a misstep early in the disease epidemic may result in an unsalable crop. If the crop is treated for Pythium root rot when black root rot caused by Thielaviopsis is really the problem, not only will time and money have been wasted but the disease will have a head start in causing damage to the crop before it can be halted with the correct fungicide. For black root rot the standard program of Cleary s 3336 rotated with Terraguard and Medallion is effective. Thielaviopsis A Team Cleary s 3336 WP/OHP 6672 FL t-methyl Medallion WG fludioxonil Terraguard 50W triflumizole Acknowledgement This research was funded in part by Floriculture Nursery and Research Initiative of the Agricultural Research Service under Specific Cooperative Agreement #58-1907-0-096 and by the American Floral Endowment.