Chapter 5: Common Diseases in Nurseries This section will cover identification of some important diseases affecting nursery crops. Classification of Plant-Pathogenic Organisms Table 5.1. Broad classification of plant pathogenic organisms Pathogen Characteristics Spread by Bacteria Fungi Mostly free living single cells or filamentous colonies Mostly filamentous (threadlike), multicellular organisms Water-borne spores or cells Air or water-borne spores Nematodes Multicellular round-worms Soil or water-borne Viruses Non-cellular, spherical or rod-shaped, obligate parasites (cannot survive outside of living cell) Mostly vectored by plant-feeding pests Common Bacterial Diseases of Ornamentals Bacteria are microscopic, single-celled organisms that do not produce visible signs. They cannot invade plant tissue directly and must enter through wounds or natural openings. Table 5.2. Common bacterial diseases of ornamentals Disease Pathogen Host Bacterial soft rot Erwinia carotovora Vegetables, bulbs, cyclamen, poinsettia Crown gall Agrobacterium tumafaciens Roses, stone fruits, grape Fire blight of pome fruit Erwinia amylovora Apple, pear, mountain ash Geranium leaf spot & stem rot Xanthomonas pelargonii Pelargonium 1
Gladiolus scab Pseudomonas sp. Gladiolus, freesia, crocus Hyacinth yellow disease Xanthomonas sp. Hyacinth Lilac blight/bacterial canker Pseudomonas syringae Prunus leaf spot Xanthomonas prunii Stone fruits Wet wood or slime flux Erwinia nimmipressuralis Lilac, magnolia, stone fruits Poplar, elm, horse chestnut, maple Common Plant Pathogenic Fungi Fungi are the largest group of plant pathogens. They are generally not visible to the naked eye although sometimes you can see their associated structures such as mushrooms or dense mycelium (e.g. powdery mildew). Fungi spread by spores carried by wind, water or animals. They enter host plants directly or through natural openings or wounds and cause damage by producing substances that change or destroy plant tissues. The main groups of fungal pathogens come from the oomycetes (water moulds), ascomycetes (sac fungi) and basidiomycetes (club fungi). Common fungal diseases include powdery mildew, rust, leaf spots, root and crown rots, stem cankers and vascular wilts. Powdery Mildew Powdery mildews come from the ascomycete group of fungi. There are many different powdery mildew fungi that affect a wide range of ornamental plants (e.g. maples, lilacs, roses, apples, zinnias, etc.). These fungi, however, are very host specific in that usually a particular species of powdery mildew fungi will only affect one type of host plant. This fungus affects aerial portions of the plant with infections commonly showing a greyish-white, powdery growth on leaves and stems. Rust Rusts are easily recognizable by their masses of yellow, orange or dark red spore masses found on leaf tissue. Some rust fungi have complicated lifecycles requiring two different host plants in order to successfully reproduce. Rusts are commonly seen affecting many ornamental plants including barberry, junipers, hollyhocks, snapdragons, hawthorns and small fruits. 2
Leaf Spot and Blight Leafspots are among the most common diseases affecting leafy tissues and there are a wide variety of plant pathogens that cause these symptoms. Some of the more common leafspots and blights include black spot on roses, apple scab and anthracnose on shade trees like sycamores, maples, ashes and oaks. Root and Crown Rots Most fungi that cause root and crown rots are among the oomycetes or water moulds (e.g. Pythium and Phytophthora). These pathogens typically affect the below ground portions of plants, damaging the root system, and resulting in poor growth, yellowing or stunting of the plant. Stem and Twig Cankers and Galls Cankers and galls are caused by a wide variety of fungi (and bacteria). These fungi usually infect woody tissues causing areas to appear raised or sunken, discoloured or split open (= canker). Some fungi will also cause abnormal swelling of woody tissues (= gall). Vascular Wilts Wilt occurs when water and nutrient flow to affected plant parts is obstructed or slowed. This can be a result of fungi or bacteria growing in the vascular tissues of plants. Infected plants will appear to be under the effects of moisture stress. Common vascular wilt diseases include Dutch elm disease and Verticillium wilt. Discoloration of the vascular tissue sometimes can be seen when you cut diagonally through a branch. 3
Common Virus Diseases of Ornamentals Viruses are submicroscopic particles that multiply only within living cells of host plants. Table 5.3. Common virus diseases of ornamentals Virus Arabis mosaic virus Aster yellows (phycoplasma) Cucumber mosaic virus (CMV) Impatiens necrotic spot virus (INSV) Prunus necrotic ringspot virus (PNRV) Tobacco mosaic virus (TMV) Tobacco ring spot virus (TRSV) Tomato mosaic virus (ToMV) Tomato ring spot virus (ToRSV) Vector / Sources of Infection Aphids Leafhopper Aphids Thrips, infected plant material Mites, s, budding, grafting, pollen, seed Sap, seed (in apple, pear and grape), tobacco products, handling Tools, handling, clothing, s, thrips, mites, seeds Soil, seed, plant and root debris, handling, unwashed clothing Nematodes Host Range A wide variety of plants, including vegetables, ornamental plants (woody and herbaceous) and weeds A wide variety of vegetables, ornamental plants (including monocots) and weeds A wide variety of vegetables, ornamental plants (including monocots, and especially chrysanthemums) and weeds Common in a wide variety of plants including floricultural crops Stone fruits (Prunus spp., including flowering cherries), roses (as rose mosaic virus) A wide variety of mainly herbaceous, dicotyledonous plants, including especially solanaceous (tomato family) plants; symptomless in apple, pear and grape A wide variety of vegetables, ornamentals, weeds and especially solanaceous (tomato family) plants Solanaceous (tomato family) plants, especially under greenhouse production Gladiolus; pelargonium; hydrangea; many weeds; raspberry; grape Tomato spotted Thrips, infected Ornamentals, weeds and solanaceous 4
wilt virus (TSWV) plant material (tomato family) plants, lettuce Common Nematode Diseases of Ornamentals Nematodes are microscopic (about 2 mm long) roundworms that live in aquatic environments (water films in soils) and in living plant tissue. They pierce plant cells with their spear-like mouthparts to feed. This reduces the vigor of the plant, as the feeding can disrupt the uptake of water and nutrients. The feeding wounds also provide easy access for fungal disease organisms to enter. Most s are soil-borne and feed on roots to complete their life cycle and reproduce. There are also several species of foliar s that can infect leaves and stem tissue. The presence of a large number of soil-borne s can be confirmed by taking a soil sample and having the s counted by a laboratory. Nematodes have a wide host range. Table 5.4. Common plant-feeding s Nematode Common name/ disease Host range Aphelenchoides sp. Foliar A wide range of ornamental and vegetable crops, including mushrooms Ditylenchus destructor Ditylenchus dipsaci Meloidogyne spp. Pratylenchus sp. Xiphinema spp. Potato rot Narcissus bulb and stem Root knot Root lesion Dagger / corky root Potato, dahlia More than 400 species of plants Many annual and perennial landscape plants (including weeds) Many crop plants and trees Many trees, woody vines, conifers, annuals 5