PLANT PROTECTION 1 â Pests, Diseases and Weeds

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PLANT PROTECTION 1 – Pests, Diseases and Weeds ‘Phytophthora’ root rot An example of a soilborne fungal disease Phytophthora, one of the world’s most damaging disease organisms affects a broad range of plant species costing millions of dollars each year in Australia. This introduced soilborne fungus became important initially because of its occurrence in the jarrah forest in WA (Keane et al. 2000, Shearer et al 2009) and the seriousness of the disease on many ornamental plants and fruit crops. Threatened species may be at high risk of extinction. Many Phytophthora species and other root rotting fungi cause major yield losses in Australia annually. Many investigative and information groups have been formed, eg Dieback Information Group www.dieback.org.au/ Centre for Phytophthora Science and Management www.cpsm.murdoch.edu.au/ Biological Crop Protection www.biolcrop.com.au/ Soilborne Diseases Symposia held regularly by the Australasian Plant Pathology Society www.apps.net.au/ Phytophthora Online Course: Training for Nursery Growers (Oregon State University, currently. available at http://oregonstate.edu/instruct/dce/phytophthora/ Scientific name Phytophthora root rot (Phytophthora cinnamomi (Pc), Phylum Oomycota) is often called ‘dieback’ but do not confuse ‘Phytophthora root rot’ caused by Pc with dieback caused by other agents, eg Armillaria root rot, Christmas beetles and other foliage-feeding insects, drought, etc. Additionally, diseases called ‘Phytophthora root rot’ may be caused by species of Phytophthora other than P. cinnamomi, eg Phytophthora root rot of lucerne is caused by P. megasperma. There are more than 60 described species of Phytophthora, many of which have been imported into Australia. Host range Wide host range, including ornamentals, eg azalea, native plants, eg Proteaceae, Epacridcaeae, Myrtaceae especially eucalypts (jarrah), susceptible commercial floriculture taxa include waxflower, banksia, boronia, crowea, rice flower, waratah, thryptomene; fruit, eg apple, avocado. peas, orange, grape, vegetables, field crops and weeds. Most states have host ranges for their state, eg Reid (2006) has provided a list of the main species of importance to horticulture in WA. Symptoms and impacts Soil diseases affecting roots and crowns are often unnoticed for years. In addition to attacking mature plants, this fungus can attack seeds and seedlings (page 371). Above ground symptoms (on shrubs, trees). Leaves may develop brown tips and margins. Generally a wilting, yellowing or dying back of foliage and a general unthrifty appearance prior to death of the plant, may be present on only one side of the plant. Damage to roots and water conducting vessels prevent plants from taking up enough water from the soil. Many of these symptoms may be caused or exacerbated by other soil diseases, nutrient deficiencies or toxicities and a range of environmental stresses, which may be operating at the same time. Plant may die during the dry summer months as diseased root systems cannot supply adequate water for plant survival. Large trees may take years to die. Collar rots and stem cankers. If the bark is removed at ground level or from stem cankers, underlying tissues are often brownish due to the fungus attacking these areas. Below ground symptoms. On removing plants from soil, affected roots are black or brown, rotted and outer areas may come away leaving a thread-like vascular system. Root system is reduced preventing uptake of water and nutrients. Tip out pots to assess root health, examine the collar region, wash roots from potting medium and examine under a dissecting microscope against a white background. Impacts. Phytophthora has been listed as a key threatening process to native vegetation in parts of Australia, whole ecosystems being affected. Many crops are seriously affected. Phytophthora spp. Many species cause damping-off of seeds, seedlings, cuttings, also root, collar and trunk rots of a wide range of plants, nursery plants. A few species attack fruit, leaves, etc. Nursery plants. P. cinnamomi Wide range of plants (native, exotic) P. cactorum Apples, pears, certain native plants P. citricola Citrus, some genera of native plants P. citrophthora Citrus, causing collar, crown, stem, root and fruit rots, also some other fruits, some vegetables, etc P. cryptogea Apples, some genera of native plants, gerbera P. drechsleri Proteaceae, many genera of native plants, nursery plants P. megasperma Wide range of plants, eg lucerne, Brassicas and other vegetables, etc P. nicotianae Many genera native plants, stone fruit, strawberry, tomato, nursery plants P. palmivora Wide range of exotic species, durian, P. infestans Late blight (Irish blight) of potatoes, tomatoes and other Solanaceae occurs in some states and some strains are still a serious disease in some parts of the world. Not discussed in this text. Fig. 204. Some of the many symptoms and diseases caused by Phytophthora spp. Combinations of causes: Fusarium oxysporum f.sp. zingiberi (Foz) and soft rot bacterium (Erwinia chrysanthemi) have played a part in the poor crop establishment of ginger in Qld. Macrophomina phaseolina and root knot nematode ( Meloidogyne incognita) play a part in root disease of chick pea. Root colonization by arbuscular mycorrhiza fungi also increases in the presence of Pseudomosas putida. 364 Fungal diseases - Examples of fungal diseases
PLANT PROTECTION 1 – Pests, Diseases and Weeds Fig. 205. Fungal. root, crown, collar rots. MOST ROOT DISEASES ARE DIFFICULT TO RECOGNIZE FROM SYMPTOMS But there are a few that are relatively easy. Get to know which root diseases your crop is susceptible to. EASY TO IDENTIFY? Those fungi that produce readily identifiable structures are relatively easy to identify, eg Sclerotium stem rot Sclerotinia rot Armillaria root rot Gray mould (Botrytis spp.) However in many situations, eg nurseries, these are not usually the commonest root rot fungi. ARE SOME MORE DIFFICULT TO IDENTIFY? Unless you have some experience with the diseases which affect your crop you will probably need to have the disease isolated and identified by an expert. Potting mixes, irrigation water and in-ground mixes may also have to be tested. Some kits are available for growers, but they are not commonly used and can be expensive. The diagnostic service will tell you how to send samples to the laboratory to prevent cross contamination of samples and spread of disease. They will identify the disease or confirm diagnosis and assist with control measures (see pages xiv, G 53). Examples of difficult to identify fungi include: Aphanomyces black root rot (Aphanomyces sp.) Ashy stem blight, charcoal rot (Macrophomina phaseolina) Chalara black root rot (Chalara sp. = Thielaviopsis sp.) Cylindrocladium root rots (Cylindrocladium spp.) Fusarium root rots (Fusarium spp.) Phytophthora root/collar rots (Phytophthora spp.) Pythium rots (Pythium spp.) Rhizoctonia stem rot (Rhizoctonia solani) Summer decline (associated with Pythium, Rhizoctonia, Drechslera, Curvularia and Fusarium) in turf Take-all (Gaeumannomyces graminis var. avenae) Vascular wilts, eg Fusarium wilts (Fusarium oxysporum), Verticillium wilt (Verticillium sp.) Other root diseases include those caused by nematodes, bacteria, and nematode-disease complexes (page 253). SYMPTOMS CAUSED BY OTHER AGENTS? Above ground, eg foliage Too little/too much water Salinity Environmental Bacterial vascular wilts Etc Below ground, eg roots. Lack of oxygen, eg due to too much water, poor drainage. Soil disturbance Etc Fruiting bodies Rhizomorphs Armillaria root rot (Armillaria spp.). Top: Clusters of mushrooms on rotting wood at base of tree trunks. Do not confuse with fruiting bodies of other fungi. Lower: Dark rhizomorphs (like on roots of infected trees are produced by some Armillaria spp. Sclerotium stem rot (Sclerotium rolfsii). Left: Tiny brown resting bodies (sclerotia) about 2-3 mm in diameter (the size of cabbage seed) on French bean. PhotoNSW Dept of Industry and Investment (M.S.Senior). Sclerotinia stem rot (Sclerotinia sclerotiorum). Left: Irregularlyshaped black sclerotia up to 12 cm long on the outside of a carrot. Right: Sclerotia formed inside infected stems. PhotoNSW Dept of Industry and Investment. Fungal diseases - Examples of fungal diseases 365
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Root structure of dock (Rumex spp.)
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