PLANT PROTECTION 1 â Pests, Diseases and Weeds

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PLANT PROTECTION 1 – Pests, Diseases and Weeds LIST OF SOME BACTERIAL DISEASES (contd) Nematode-disease complexes Free-living nitrogenfixing bacteria include endophytes, plant growth promoting Rhizobacteria and saprophytes. They find their own energy source to convert into nitrogen Nemacur biodegradation. These bacteria also impact on the winter survival of certain insects and on weather systems COMMON NAME 298 Bacterial diseases SCIENTIFIC NAME (alphabetical order) Xanthomonas campestris pv. begoniae HOST RANGE (not exhaustive) Begonia Bacterial wilt and leaf spot of begonia Black rot of crucifers (S*) X. campestris pv. campestris Crucifers Bacterial blight of stock (S) X. campestris pv. incanae Stock Bacterial blight of walnut X. campestris pv. juglandis Walnut Bacterial leaf & stem rot of pelargonium X. campestris pv. pelargonii Pelargonium. Common blight of bean X. campestris pv. phaseoli French bean, navy bean, (S) some other beans Bacterial spot of stone fruit X. campestris pv. pruni Prunus spp. especially plum Bacterial leaf spot of zinnia (S) X. campestris pv. zinniae Zinnia elegans Angular leaf spot of strawberry X. fragariae Strawberry Annual ryegrass toxicity (ARGT) (nematodes page 253) BENEFICIAL BACTERIA Nogall Agrobacterium sp. Crown gall bacteria Formulations of Bacillus thuringiensis (Bt) are available to control some species of leafeating caterpillars. The caterpillars eat the bacterial spores which contain a toxin that causes septicaemia and death. Caterpillars with a high gut pH are susceptible. Novodor® Bacillus thuringiensis (Bt) var. tenebrionis Chrysomelid and tenebrionid beetles in eucalypt plantations Cybate®, Vectobac® Bt var. israelensis Mosquitoes Dipel® Bt var. kurstaki Some leafeating caterpillars, mosquitoes XenTari® Bt var. aizawai Caterpillars, eg corn earworm, diamondback moth Cybate®, Vectobac® Bt var. israelensis Mosquitoes Bt crops, eg cotton Cotton Helicoverpa caterpillars Bt bringai India Brinjai (eggplant) Fruit and shoot borer in India Being researched in Australia to control dengue fever Nitrogen-fixing bacteria Researchers aim to transfer the same traits to cereals, eg International Rice Nodulation Group Endophytic bacteria Some strains can fix large quantities of N in a crop cycle Plant Growth Promoting Rhizobacteria (PGPR) Promote root growth, enabling greater nutrient uptake, superior plant growth and higher yields, and in legumes, optimal N fixation Bio-Stacked Companion. Soil inoculant in horticulture and agriculture Saprophytic bacteria Biodegradation, bioremediation Epiphytic bacteria on the foliage of plants reduce frost damage. May increase growth and disease resistance Wolbachia bacteria Rhizobia spp. Azospirillum spp., in wheat roots, turf Azobacter spp. Pseudomonas spp Bacillus subtilis (various strains have been studied for use as bio-control agents) Bacillus subtilis (various strains), may be formulated with fungi, eg Trichoderma Bacteria (also fungi, nematodes, flies, etc) Naturally-occurring bacteria (various species) Various bacteria, eg strains of Pseudomonas fluorescens h BACTERIA THAT CAN AFFECT HUMANS Do not confuse with Legionella, Legionella longbeachae L. pneumophila Legionnaire’s disease associated with inhalation of water droplets from Salmonella Escherichia coli contaminated cooling towers Whooping cough Bordetella pertussis (S) Indicates that the disease is seedborne Kills the dengue fever mosquito (Aedes aegypti) before it can pass the dengue fever virus to humans Legumes convert N in the air (not accessible to plants) into a form (ammonia) that plants can use to make proteins Live in the intracellular spaces in plant vascular systems and take dissolved gas from the sap flow and covert it into amines and ammonium nitrogen for plant use Live on the surface of plant roots in the soil, consuming sugary exudates from the plant; they use this energy source to fuel the conversion of N gas into plant available N. Suppresses Fusarium, Pythum, Phytophthora, Rhizoctonia, in protected environments Breakdown plant residues, stubble, organic matter, compost Break down pesticides, eg Nemacur (fenamiphos), sulfur, pollutants, nutrient residues Bacteria lower the temperature at which ice forms by several degrees, does not provide against a dramatic drop in temperature. Pneumonia in humans, potting mixes, water fogging systems Food spoilage, food poisoning Highly contagious
PLANT PROTECTION 1 – Pests, Diseases and Weeds NUTRITION AND PARASITISM HOW BACTERIA INFECT HOST PLANTS Bacteria cannot physically penetrate protective barriers of plants Most bacteria parasitic on plants develop on host plants as parasites on the plant surface, especially on buds as epiphytes and partly in plant debris or in the soil as saprophytes (Agrios 2005). Under suitable conditions, they can become parasitic. Most plant pathogens are facultative saprophytes and can be grown artificially on nutrient media (page 324). NATURAL OPENINGS There are always some bacteria on plant surfaces. Some never harm the plant; others which cause disease can infect the plant through natural openings (leaf stomates, leaf scars, lenticels, small pores at the margins of leaves (hydathodes), or through relatively fresh wounds (hail, pruning wounds, etc). Bacteria that cause leaf spots randomly on the leaf surface are likely to have invaded leaves through stomates WOUNDS Damaging the intact surface of a plant can facilitate the entry of bacteria. DISTRIBUTION WITHIN A PLANT Some bacteria destroy the material holding plant cells together, plant cells collapse causing sunken areas on stems, tubers or bulbs, eg soft rots which may be accompanied by unpleasant smells, eg soft rot of potato. Some remain mostly on the surface of plant tissue, eg galls, and tend to decrease in numbers as they invade the gall tissue, eg crown gall bacteria. Bacterial cells may invade water and food-conducting tubes of plants: – Spreading quickly to fruits and seeds. If these are used to produce a new crop, bacteria will quickly produce diseased seedlings which may die. – Bacteria also multiply in, and block, water-conducting cells causing wilting of plants parts above the blockage (page 296, Fig. 149). DISEASE CYCLE Many bacteria parasitic on plants develop partly on the host as parasites and partly in the soil, or on plant debris in the soil, as saprophytes. HOST ONLY These diseases produce their populations on the host plant. If the bacteria do reach the soil, eg via fallen leaves or fruit, their populations rapidly decline and they do not play a part in the spread of the disease, eg bacterial blight of walnut, bacterial canker of stone fruit. These bacteria have developed sustained plant to plant infection cycles. HOST, HOST DEBRIS AND SOIL These diseases build up large populations on the host. If the bacteria reach the soil, through decaying parts of the plant, they can remain there for many years, populations only gradually declining over years. Susceptible plants in contaminated soil will soon become infected, eg crown gall. HOST, HOST DEBRIS, BUT MAINLY SOIL. These diseases produce most of their populations by growing on plant matter in the soil. They only attack plants incidentally. Plants are not essential for their continued existence, eg bacterial soft rot of iris or potato. Bacterial diseases 299
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Root structure of dock (Rumex spp.)
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PLANT PROTECTION 1 â Pests, Diseases and Weeds