 Anthracnose on lupine.Herbaceous perennials are tough plants and not disease-prone. When crowded together in a greenhouse or nursery and irrigated from overhead, however, these plants can have major disease outbreaks if preventive measures aren’t in place. Disease management is easiest when you are well prepared.Botrytis blights Greenhouse growers are familiar with Botrytis cinerea, which causes the common gray mold that blights flowers indiscriminately and attacks stems of more vulnerable crops like lisianthus and geranium. With perennials, some additional Botrytis species are relevant.  Leaf spots caused by Botrytis elliptica on Asiatic hybrid lilies are oval and brown.B. elliptica causes oval brown spots on hybrid lilies. In rainy landscapes, the lower leaves of lilies can be entirely scorched. Peonies are prone to B. paeoniae, which blasts foliage, stems and flower buds in wet conditions. Even tulips are prey to their own Botrytis species. B. tulipae, which travels as sclerotia on bulbs, causes distortion of foliage and petal blight when temperatures are cool (optimum 55°F) and wet conditions occur. Avoid extended periods of leaf wetness by spacing plants, using fans, installing wire mesh benching, and greenhouse humidity management. Fungicides including Camelot, Daconil, Decree, Chipco 26019, Pageant, Palladium and Phyton 27 can help to manage Botrytis at times when the environment cannot be controlled.  Leaf spots Leaf spots may be caused by fungi, bacteria, viruses, foliar nematodes, nutrient problems or chemical spray phytotoxicity. A diagnostic lab may be needed to determine what disease is present. Virus symptoms due to impatiens necrotic spot virus or tomato spotted wilt virus may become more common as the thrips vector becomes increasingly difficult to control. Fungi and bacteria causing leaf spots are very host-specific, so that they tend to infest only one crop at a time. Beware of Xanthomonas leaf spot on perennial geraniums. This disease is caused by the same organism (Xanthomonas hortorum pv. pelargonii) that can infect florist’s geraniums (Pelargonium spp.). On both bedding plants and perennials, the bacteria cause small round brown spots about 1/8-inch in diameter. Only florist’s geraniums also develop a systemic wilt. Keep perennial geraniums well away from other Pelargonium crops. Fungal leaf spot diseases occur on many individual perennials, but usually present only minor problems. One especially damaging disease is anthracnose on lupines caused by a Colletotrichum species. Round or half-moon tan leaf spots may be accompanied by a twisting of petioles or drooping of leaves at the end of the petiole. Sometimes the orange ooze of fungal sporulation is visible on the spots or stem cankers. Colletotrichum can be spread on seed. It is important to monitor plug trays for the first indication of an infection. Symptoms may become obvious only on older plants, when conditions are warm and wet. Physically removing diseased plants and protecting the rest with fungicides is the best course of action. The most difficult to control leaf spot currently is Septoria leaf spot on Rudbeckia ‘Goldsturm.’ Plants should be treated early in the growing season using a rotation that includes systemic and contact materials. There are many fungicides with excellent protective effects against fungal leaf spots, including strobilurins such as Compass, Cygnus, Heritage, Insignia and Pageant, as well as the older myclobutanil (Hoist, e.g.), chlorothalonil (Daconil, e.g.) and mancozeb (Protect, e.g.) materials. Minimizing leaf wetness always helps. Rusts Rusts are becoming increasingly apparent on perennials. Growers should try to avoid growing the most susceptible cultivars. Also, watch carefully for the beginnings of rust outbreaks so that treatments can be effective. Recently heuchera, veronica, campanula and switchgrass have shown severe rust infestations. Reducing leaf wetness periods helps to reduce rust development. Heritage and other strobilurins are especially effective for rust control. Mancozeb is good as a protectant. Materials in the demethylation inhibitor (DMI) category including Strike, Terraguard, Hoist are helpful. Propiconazole (BannerMAXX) should be added to the control arsenal for outdoor use. Powdery mildew The disease most often associated with herbaceous perennials is certainly powdery mildew. Monarda and phlox are some of the most mildew-prone plants. Although they all look similarly white and powdery, most powdery mildew fungi are somewhat host-specific. Powdery mildews are best managed by choosing cultivars that are less susceptible. Phlox ‘David’, the 2002 Perennial Plant of the Year, has forged a fine reputation as a white-flowered phlox with less vulnerability to this disease. Always be on the lookout for phlox and monardas that maintain reasonably green foliage under your local climate conditions. A wide variety of fungicides can reduce the whiteness of foliage, including treatments with biofungicides (Actinovate, Cease, RootShield) or potassium bicarbonate (MilStop), as well as systemic treatments such as strobilurins (Compass, Pageant, etc.) or demethylation inhibitor materials (Hoist, Strike, Terraguard, etc.). BannerMAXX may be used outdoors.  Downy mildew usually forms the “down” of its sporulation on the undersurface of rudbeckia leaves. Under very humid conditions, white sporulation may be evident on the upper surface of the leaves as well.Downy mildews One group of diseases that has risen in prominence in the past decade is the downy mildews. The pathogens are not true fungi, but are relatives of Pythium and Phytophthora. Downy mildews usually cause angular leaf spots. Some also form systemic infections that lead to stunted, often chlorotic new growth. Veronicas often show distorted, stunted, pale growth in the spring due to systemic downy mildew. On many crops, downy mildew diseases cause discolored leaf patches, as on lamium, agastache, geranium, geum and rudbeckia. Mancozeb (e.g. Protect) and coppers (Camelot, etc.) are good contact-action protection against downy mildew. To get systemic protection, use dimethomorph (Stature SC), mefenoxam (SubdueMAXX, etc), phosphorous acid materials (Aliette, Alude, K-Phite, Vital, etc.) and strobilurins (Heritage, Pageant, etc.) or Fenstop. Be careful to follow label directions regarding rotation and tank mixes.  Foliar nematode infestations of Japanese anemone cause blocky patches of dark discoloration in the leaves.Foliar nematodes It is easy to be fooled by foliar nematodes, since their leaf spotting is very similar to that caused by downy mildews. The lesions are bounded by veins because the nematodes find them to be barriers to their movement across the leaves. Some crops (e.g. Japanese anemone and bergenia) are notoriously susceptible to foliar nematodes. These cannot be cured or even properly protected with chemical application, so remove lower foliage showing symptoms and water sensitive crops at the soil line.  Plants attacked by Phytophthora crown rot, as shown here on lavender, will wilt and die.Phytophthora and Pythium The main pathogens affecting perennial roots are usually Phytophthora or Pythium species. Poor drainage and over-fertilization lead to root stress that is taken advantage of by these water molds. Lavender is especially vulnerable to root and crown rot, showing wilted foliage due to attack at the root crown by Phytophthora nicotianae. Plant species touted for their ability to withstand drought may be especially intolerant of excessive moisture during production, so irrigation should be adjusted accordingly. Fungicides for protection against Pythium include Truban, Terrazole, Segway, SubdueMAXX and Banrot. There are strains of Pythium that are resistant to SubdueMAXX. Truban, Terrazole and Banrot share one active ingredient. Biofungicides (e.g. Cease, RootShield) may be used preventively against Pythium as well. Phytophthora has a larger array of effective control materials, starting with those that work well against Pythium. Also effective against Phytophthora are phosphorous acid materials, strobilurins and Stature SC. Rhizoctonia and Thielaviopsis True fungi can attack perennial roots. Stunted growth on leucanthemum, gaillardia, astilbe and other crops has been caused by black root rot (Thielaviopsis basicola). This fungus is controlled by using a low growing medium pH (below pH 6.0) and applying fungicides including thiophanate-methyl, which is found in Cleary 3336, OHP 6672 and Banrot, and is especially effective. Thiophanate-methyl also protects against Rhizoctonia canker, which is occasionally a problem on dianthus and other plants with dense crowns that hold high humidity around the stem. Other control materials for Rhizoctonia canker include strobilurins, Chipco 26019, Medallion and Terraguard, as well as biofungicides. Good sanitation practices are important for management of both of these root rot pathogens, which form resistant structures that can linger in pots or flats or on debris from previous crops. If containers must be recycled, it is important to remove organic debris and treat them with a disinfectant. Margery Daughtrey is senior extension associate, Cornell University, Department of Plant Pathology and Plant-Microbe Biology, (631) 727-3595; mid9@cornell.edu. She recently co-authored the book “Diseases of Herbaceous Perennials,” www.apsnet.org/apsstore/shopapspress. |
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