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Introduction to Plant Diseases Damon L. Smith Assistant Professor and State Extension Specialist Department of Entomology and Plant Pathology, Oklahoma State University Plant Disease Plant Disease: any malfunctioning of the growth or reproduction of a plant caused by continuous irritation by a primary causal agent that results in the development of symptoms Plant Disease Continuous irritation : Very important point; this separates disease from injury • Injury = acute • Disease = chronic Broad classification of Primary Causal Agents • Biotic (infectious): living organisms (pathogens) that have the ability to infect plants and cause disease • Abiotic (non-infectious): disorders induced by non-living causes such as environmental conditions (85% of our lab samples are identified as abiotic disorders) Infectious Disease Agents (Pathogens) • FUNGI: cause the great majority of plant diseases; thousands have been reported • BACTERIA: six genera are known to cause plant diseases • VIRUSES: Submicroscopic obligate parasites • NEMATODES: small non-segmented worms Fungi • • • • • • • • Most common of plant pathogens Attack all plant parts About 16,000 fungal plant pathogens Unit of infection = spore or mycelium Multi-celled Visible – microscopic E.g., cedar-apple rust, black spot of rose Spread by wind, insects, water, tools Fungi FUNGI grow apically by linear extension of their threadlike bodies from the tips. A single strand of this fungal body is termed a hypha (plural hyphae), and the hyphae taken all together are called a mycelium. Nuclei are found in the hyphal cells. Multinucleate hypha Most fungal hyphae have crosswalls; some do not. Mycelium on leaf surface Examples of types of fungal conidia (asexual spores). These conidia are microscopic and easily carried by air currents. Botrytis Verticillium macroconidia microconidia chlamydospore Aspergillus Pencillium Fusarium Fusarium Spore germination and infection • Spores require high humidity for development (>80%) • Spores require water film on the host tissue to germinate • Many germinated fungal spores penetrate plant tissue directly, or enter through the leaf pores (stomates) or wounds. • Most conidia can survive from several hours to several days if conditions necessary for germination are not met. • Resting spores with thick walls may survive for years. Bacteria • • • • Quite small and single celled Spread by water, tools and seed (sometimes) Enter wounds or natural openings E.g., soft rot of vegetables, fire blight, crown gall Bacteria Crown gall of grape Bacterial scorch of oak Bacterial cells grow by dividing into two cells from one (called binary fission) Bacterial cell Dividing cells How Bacteria Spread Plant pathogenic bacteria do not form spores Bacteria are spread by: • Water splash and runoff • Insects (honey bees, insect larvae, chewing insects, etc.) • Tools (such as pruning tools, trowels) • Contaminated flats, pots, or soils (in greenhouses etc.) • In contaminated or infected seed • By cuttings taken from infected plants, or in infected planting stock (tubers, bulbs, etc). • Not spread by wind (exception of wind-driven rain drops) How bacteria gain entry into plants Bacteria cannot penetrate plant tissue directly Bacteria usually enter plant tissue by means of: • Wounds • Natural openings on plant leaves or stems: lenticels, hydathodes, etc; Water saturation aids entry through natural openings. • Insects or insect larvae are common vectors of bacterial pathogens. • Bacteria may “piggy back” and gain entry through cankers (open wounds) caused by fungi. Viruses • Very small, need electron microscope to see • Spread by insect, seed, contact (handling, propagation), nematodes, root grafts Virus Diseases of Plants Viruses = nucleic acid (RNA or DNA) surrounded by a protein coat. Viewed with electron microscope. Shapes include: short rods, long flexuous rods, or polyhedrons. Exist and multiply only in living tissue. Transmitted from plant to plant 1. insects (mostly aphids and leafhoppers) 2. mechanically through small wounds 3. parasitic plants like dodder SYMPTOMS CAUSED BY VIRUSES 1. Rarely kill plants 2. Some degree of stunting of entire plant 3. Symptoms usually systemic (throughout the plant) rather than localized 4. Symptoms usually appear on leaves as a. mosaic (yellow areas intermingled with normal green areas) b. ring spots (chlorotic or necrotic rings) c. line patterns (chlorotic or necrotic lines) d. lighter colored areas mingled with normal color on flowers or fruit. e. yellows (overall chlorosis) f. tumors g. pitting or flattening of stems or fruits Viruses http://www.oznet.ksu.edu/dp_hfrr/extensn/pr oblems/tomswv.htm TSWV on Tomato Leaf roll virus of grape TSWV on Peanut Nematodes • • • • Non-segmented, round worms Most feed on roots; some on foliage & stems Usually microscopic E.g., root-knot nematode Nematodes • 15,000 described species. Size ranges from 3/1000th of an inch to 27 feet in length. -Most are nonpathogenic, but several hundred species (10% of described species) are parasitic on living plants. -Plant parasitic nematodes typically measure from 1/25th to 1/50th of inch in length • Females lay eggs that hatch into small nematodes. • Characteristically cause stunting of plants, may also cause cysts, galls, or lesions on roots. • Spread by anything that moves soil. i.e. wind, water, tools, animals, man Plant Parasitic Nematodes Root Lesion Nematodes Endo-migratory Cyst Nematodes Ecto-sedentary Root Knot Nematodes Endo-sedentary Ring nematode Sting Nematode Ecto-migratory Spiral Nematode Plant pathogenic nematodes have hollow stylets or spears as mouthparts with which they withdraw their nutrients from plant cells. Root Knot Nematode: Meloidegyne incognita Stylet (spear) Symptoms Resulting from Nematode Feeding Root knot nematode G Root knot nematode; two small, individual galls (G) G Watermelon: advanced stage, heavy infection Plant Disease Management Principles and General Concepts Plant Disease Triangle Host Pine Trees (many species) Diplodia Tip Blight Pathogen Fungus: Diplodia pinea Environment Wet Conditions; cool to moderate temperatures Manipulate the Host Component • • • • Genetic resistance most powerful disease management tool Use native plants (fewer disease problems) Select ornamentals and trees adapted to Oklahoma conditions (Oklahoma Proven Varieties) Plant stress management – Stress predisposes plants to disease Host Pathogen Environment Manipulate the Environment • Do not over fertilize – many diseases are more severe when there is an excess of nitrogen available • Reduce competition for water, light, nutrients • Provide adequate air movement and light penetration • Irrigate in morning • Apply irrigation water to the soil (e.g. drip irrigation) Host Pathogen Environment Reduce Amount of Pathogen • Exclusion – Preventing the introduction of a pathogen (beware of exotics!) • Eradication – Constant observation and early diagnosis is critical • Sanitation – Removing pathogen infested plant material from the area • Pesticide Use Host Pathogen Environment Fungicides • Protectant/contact – remain outside the plant and only protect the tissues to which they are applied – chemical does not move after it dries – removed by pruning, often degraded by the environment – Mancozeb (Fore, Formec) • Systemic – chemical moves through the plant – slightly misleading – fungicides move upward – only true systemic is fosetyl-aluminum (Aliette, Flanker, etc.) • Local systemic (penetrant)/mesotemic – Triazole and strobilurin chemistries – absorbed into tissues but have limited movement – Propiconazole (Propiconazole, Fertilome Systemic Fungicide, Banner), trifloxystrobin (Compass) Fungicide Resistance Management •Fungicide resistance can be a problem if MOA rotation is not used, proper spray volume is not used, off-label rates are used, etc. Recommended Fungicide Use Patterns = Sensitive Individuals Non-recommended Fungicide Use Patterns = Resistant Individuals Accurate Diagnosis • Submit sample to OSU Plant Disease Diagnostic Lab • Proper identification will lead to the right management methods • Send a good sample! Jennifer Dominiak-Olson, Plant Pathology Diagnostician Contact: Room: 119 NRC Phone: (405) 744-9961 FAX: (405) 744-6039 http://www.ento.okstate.edu/pddl/