PLANT PROTECTION 1 â Pests, Diseases and Weeds

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PLANT PROTECTION 1 – Pests, Diseases and Weeds NUTRITION AND PARASITISM Fungi and bacteria are alike in one respect, they have no chlorophyll. Fungi obtain their food either by: Infecting living organisms such as plants as parasites, or Attacking dead organic matter as saprophytes. OBLIGATE PARASITES Fungi which can only attack and complete their life cycle in nature on living host plants as parasites, eg downy mildews, powdery mildews, rusts (few exceptions). Host specificity in some cases is extreme. FACULTATIVE PARASITES Fungi which can live for an indefinite period on dead organic matter as saprophytes. When host plants are available and environmental conditions are favourable, they become parasites, eg damping-off fungi, Phytophthora root rot, Botrytis, Verticillium. Some fungi can grow actively only on debris from the host plant. Others can live for long periods by obtaining nourishment from dead leaves and other plant material, only attacking living plants when they are available. OBLIGATE SAPROPHYTES Fungi which can live and complete their life cycle only on dead organic matter as saprophytes,eg mushrooms, wood rot fungi. Wood rot fungi attack the dead parts of a tree. FACULTATIVE SAPROPHYTES Fungi which spend most of their life cycle on living plants as parasites and may survive as saprophytes for short periods of their life cycle, eg smuts. HOW FUNGI INFECT HOST PLANTS Fungi enter host plants by several means including: NATURAL OPENINGS The germ tubes of some fungi, eg downy mildews, usually only penetrate host plants through natural openings such as stomates and lenticels. Germ tubes grow through open stomates and lenticels MECHANICAL PRESSURE The hyphae of some fungi, eg storage moulds and root rots, penetrate host plants by using mechanical pressure. WOUNDS Some fungi can enter plants through damaged surfaces. Bruised oranges are susceptible to Penicillium rots. Damage to trees by lawn mowers facilitates infection by wood rotting fungi. DISTRIBUTION WITHIN HOST PLANTS Hyphae produce enzymes which change plant tissue into a food source HYPHAE Hyphae of some fungi, eg powdery mildews, grow on the plant surface, sending haustoria into surface cells to obtain nourishment. The furry or powdery growth on the surface of plants is composed of hyphae and spores. Hyphae of other fungi grow inside plants, eg Fusarium wilts grow inside xylem vessels of infected plants. Some downy mildews grow systemically within plants. Endophytic fungi also grow systemically within plants but cause no disease symptoms; they may improve resistance to certain pests, diseases, drought and heat. Regardless where mycelium grows in the host, spores are produced at or near the surface of the host ensuring their prompt dispersal. Many mycorrhizal fungi produce their spores underground and rely on fungal-feeding animals, eg marsupials and insects (‘earth boring’ beetles) for their spread. 324 Fungal diseases - Biology, identification and classification
PLANT PROTECTION 1 – Pests, Diseases and Weeds DISEASE CYCLE ALMOST ALL FUNGI WHICH INFECT PLANTS spend part of their lives on host plants and part in the soil, or in or on plant debris in the soil. HOST ONLY These fungi spend all their vegetative life cycle on the host plant. Spores may land on soil or plant debris where they remain until carried to a host where they can germinate, grow and complete their life cycle, eg powdery mildews. HOST AND HOST DEBRIS These fungi grow parasitically on their hosts and continue to grow on the dead tissues of their hosts as saprophytes to complete their life cycle. They can only grow on the organic matter of their host, eg apple and pear scab. HOST, HOST DEBRIS, OTHER DEBRIS AND SOIL These fungi grow parasitically on their hosts but continue to grow on the dead tissues of the host after it has died. These fungi also grow out of the host plant into the soil or other decaying plant material, where they can grow and multiply as a saprophyte, eg Sclerotinia, Sclerotium. WHY IS KNOWLEDGE OF THE DISEASE CYCLE IMPORTANT? Knowledge of the disease cycle is essential for the implementation of effective control measures, including: Timing of pesticide applications. Application of preventative protectant treatments. Whether leaves should be gathered and destroyed. Whether seed from diseased plants should be used. OVERWINTERING, OVERSUMMERING Resistant spore Rhizomorphs Sclerotia CARRYING OVER THE FUNGUS FROM ONE SEASON TO THE NEXT Fungal structures. Hyphae may grow into different structures which ensure survival and spread of the fungus but which make control difficult. Dormant stages are stimulated into growth when the roots of a susceptible host plant is in close proximity and conditions are favourable, eg – Thick-walled spores, eg chlamydospores, oospores of Phytophthora. – Sclerotia consist of a mass of hyphae bunched together formed by some fungi, eg Rhizoctonia, Sclerotium, Sclerotinia. Sclerotia are pale at first but they darken as the hyphae on the outside dry out forming a hard skin that protects the hyphae inside, enabling them to start growing again sometimes years later when conditions are favourable. They are formed either inside or on outside of plants. – Rhizomorphs are bundles of parallel hyphae (about the thickness of a shoelace) formed by some fungi, eg Armillaria, that can grow through the soil to new hosts. However, Armillaria does not appear to readily for m rhizomorphs in Australia. ON OR IN THE HOST PLANT Bud scales - Peach leaf curl of stone fruit Twigs, branches Trunks - Brown rot of stone fruit, powdery mildews - Wood rots Roots - Phytophthora root rot, Rhizoctonia stem rot ON OR IN PLANT DEBRIS Leaves Fruit - Apple scab - Brown rot of stone fruit Trunks - Wood rot Roots - Phytophthora root rot SOIL As mycelium, sclerotia or thick walled resting spores, eg Phytophthora, Rhizoctonia, Fusarium and Verticillium. SEED Many fungal diseases are seedborne. They are either within or on the surface seed, eg rust spores adhere to the outside of seed of infected plants. ‘OVERWINTERING’ IN SEVERAL WAYS Some fungi can overwinter in several ways, eg Phytophthora root rot on host plants, in plant debris, soil and as thick-walled spores. Botrytis blight as spores on leaves of rooted cuttings, discarded plant debris, etc. Fungal diseases - Biology, identification and classification 325
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Root structure of dock (Rumex spp.)
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PLANT PROTECTION 1 â Pests, Diseases and Weeds

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PLANT PROTECTION 1 â Pests, Diseases and Weeds