Plants Are Victims Too

Plants Are Victims Too


Microoganisms were here from the beginning and effect all aspects of life. One might think they have brains considering how they attack plant mechanisms and defy human technology.This is a non-technical overview how bacterial plant pathogens behave and evolve to become a menace to plants and the human food chain. They manage to survive in all climates.


Bacterial blight of anthurium (Xanthomonas axonopodis) can cause chaos to the flower industry in tropical areas and problems in temperate climates. It spread from New Jersey in 1932 to many areas in the world. Controls consist of using disease free stock and sanitation methods. Might be fun to check this disease out in Hawaii, a major region in the world for growing anthurium flowers.


The electron microscope shows flagella that bacteria use to navigate when moisture is present. Bacteria can easily enter pear and apple blossoms on a rainy day.

Credit: Anne K. Vidaver and Patricia A. Labrecht


Fire blight is the common name of the disease, and Erwinia amylovora is the pathogen. It is a very versatile bacterium making it a tricky adversary to control. It is motile, senses natural openings in plants, can float in the air, and produces ooze in trees. This ooze attracts bees and other insects that carry bacteria to susceptible plants including the common Pyracantha.  The bacterium is reported to infect 75+ kinds of trees and shrubs. It can be devastating in certain climates in California. 


Here today and gone tomorrow. This bacterium (Pseudomonas syringe) multiplies in cold weather. A vigorous young plant in autumn such as apricot and related “stone fruits” may die by spring.  It infects trees in all soil types and in many climates contrary to some reports.  There is no good cure. The common name is bacterial canker but it has other names such as dieback and blight. The bacterium has a wide host range from trees to landscape plants. One never knows when it may show up and cause death or serious damage. 


Bananas may be the world’s most important fruit crop considering production, volume, and trade. It is susceptible to many bacterial diseases with names such as moko/bugtok, blood disease, Xanthomonas wilt, and head rot. The most serious disease of all is caused by Ralstonia solanacearum with its many strains.


Halo everybody. This disease called halo blight is common in moist areas. It rarely is found in California because of the dry climate.  However, being seed borne it could show up if grown in a humid green house. Its scientific name is Pseudomonas syringae pathovar (pv.) phaseolicola.  It has many strains making it difficult to control and keeps plant breeders busy in developing resistant plants.


Watch out! Watch out! Citrus greening (‘Candidatus Liberibacter asiaticus’) threatens citrus everywhere.  It is the most dangerous disease of citrus in the world and has already devastated crops in Florida.  The insect vector is already in California.   If the bacterium shows up, the two together cannot be stopped as there is no control.   


Citrus canker disease (Xanthomonas citri subsp. citri) is not a killer but damages trees by defoliation of leaves and twigs. It might have been controlled if eradication of infected trees had not been impacted by politics and legal delays. This lack of action is common throughout the world. 


Citrus stubborn disease (Spiroplasma citri) occurs in warm areas but is not as threatening as with other bacterial diseases. The disease causes low yields and stunted growth. Leafhoppers and contaminated grafts of young trees spread it.


Known as lethal yellows (‘Candidatus Phytoplasma palmae’) this is one lethal disease!  It is found in the Caribbean, Africa and is spreading in Florida. Insects such as leafhoppers disperse it. There are some semi resistant varieties.


Little was known about corn stripe (Burkholdearia andropogonis) when this project began although it was observed in 1973-1978 in various states. It has a large host range such as sorghum, Johnson grass, white clover and chickpea.  There is a problem of diagnosis as it looks a lot like Stewart’s wilt and Goss’ wilt.


Stewart’s wilt (Pantoea stewarti subsp. stewarti) can be a serious disease of corn. This bacterium is a great example how microorganisms can find unique ways to survive. Rather than staying in the freezing soil, it overwinters in the gut of corn flea beetles and would die otherwise.


“King Cotton” suffered from blight before the Civil War. The disease known as bacterial blight of cotton (Xanthomonas citri subsp. malvacearum) was finally described in 1891. It has many strains that keep popping up. This diversity makes it difficult to control. The disease is far worse in rainy years and periods of high humidity. 


(Brussels sprouts, kale, cauliflower, cabbage and similar leafy vegetables) This bacterium loves cabbage. For over 100+ years the disease known as black rot has challenged farmers to control it. Its scientific name is Xanthomonas campestris  pv. campestris.


Do you like blotchy watermelons? A new bacterium named bacterial fruit blotch (Acidovorax avenea subsp. citrulli) was discovered in the US in 1965 and is spreading. This is another example how new diseases keep showing up.  Governments need to be more responsive before such diseases spread making it too late to control. 


Bacteria wilt of cucurbits (Erwinia tracheiphila) is “peculiar” in attacking only some members of the cucumber family such as squash, cucumber, and gourd. It also is clever. It shuns the freezing soil and overwinters in the digestive track of two different beetles. 


Not edible if this bacterium has its way. Known as bacterial wilt of ginger (Ralslstonia solanacearum race 4) it stunts the rhizome and can cause it to rot with help from another bacterium, Erwinia sp. 


Keep it out! Keep it out! Bacterial blight of grapevine (Xylophilus ampelinus) is found in Europe, Africa and Australia and is easily spread by tools.  It is controlled by sanitation methods. 


The olive knot bacterium (Pseudomonas savastanoi  pv. savastanoi) produces a growth hormone, indole-3-acetic acid, that causes knots on all parts of the tree.  The bacterium spreads by rain and contaminated propagation twigs.  There is no evidence that this bacterium kills trees as reported. By far the most dangerous olive pathogen in the world are strains of Xylella fastidiosa that are killing thousands of olive trees in Italy and other parts of Europe with the help of an insect vector (spittle bug). This disease has different strains and has not caused the havoc in the US as in Europe. In fact, Pierces disease of grape in the US is caused by X. fastidiosa.


St Croix (U .S. Virgin Islands) field station had a serious papaya disease as did many other countries.  M. Schroth asked to visit and was denied on the basis “they were tired of experts coming and not finding an answer”.  Apparently, no plant bacteriologist was ever invited. With support from the USDA, Schroth visited the station in 1980 and swabbed some ooze from sick trees on a Petri dish medium. Three days later there were pure cultures of a bacterium. The common name is bacterial canker. Scientific name is Erwinia papayae.


Health experts say pineapple juice is good for the heart, but ironically this fruit also suffers from a disease called bacterial heart rot (Dickeya sp). 


Ring rot is caused by a well-researched bacterial pathogen, Clavibacter michiganensis subsp. sepedonicus.  This bacterium survives primarily when there is poor sanitation. It can survive in rubble, potato sacks, machinery, and unharvested infected potatoes that are still in the soil waiting to grow the next season.


Scabies diseases are problems for both humans and potatoes. The difference is Streptomyces scabies infects potatoes whereas mites irritate humans causing scabs. Both disease agents can live without a host as they have other food sources.  When one thinks about it, there is much in common in the ecology of human and plant bacterial pathogens.  For example, strains of the common bacterium, Pseudomonas aeruginosa, infect both humans and plants. Also, both humans and plants can be symptomless carriers. This is not uncommon for microorganisms.  P. aeruginosa can be a deadly disease in hospital burn units.


Potato brown rot disease (Ralstonia solanacearum) has many strains and attacks various plants throughout the world. The two strains that attack potato and geranium (southern wilt) are classified as race 1 (R1) and race 3 biovar 2 (R3bv2). Taxonomic jargon is needed for communication when dealing with microorganisms. 


Bacterial leaf streak (Xanthomonas oryzae pv. oryzicola) was not in the US during the making of this website. It is a serious disease in many parts of the world and also infects some cereals. 


Millions of hectares of rice are infected annually by a disease called bacterial rice blight (Xanthomonas oryzae pv. oryzae). It produces white ooze at first that later turns yellow. 


(potato, tomato, nightshade, petunia, tobacco, pepper etc.) How is it possible that this deadly bacterium (Ralstonia solanacearum) is not in California? It has to be something to do with the environment? The family Solanaceae has 102 genera and 2,500 species of plants with a great number being  susceptible to the many strains of this pathogen. 


Like strawberries? So does this bacterium with the common name angular leaf spot (Xanthomonas fragariae.)  Sanitation methods and growing stock in safe areas fortunately can control it. 


This disease named leaf scald (Xanthomonas  albilineans) is an apt description for sick plants.  It is a sneaky type of bacterium.  It can be inside plants and show no symptoms, hiding for months and occasionally years.  It was found in Florida, 1967.  It is spread many ways by airborne floating diseased leaves, contaminated seed, and tools. High humidity greatly increases severity of the disease. It  occurs in at least 60 countries. Fortunately, there are degrees of resistance thanks to plant breeders.


Gardeners and farmers in wet climates worry about the many diseases of tomato. The diseases known as tomato canker, speck, and spot are serious and easily diagnosed. Whereas those diseases are not common in California because of the dry climate, they can pop-up when growing tomatoes in the back yard in a moist warm situation. When this happens, it usually is because of contaminated seed.


Shallow and deep bark canker diseases and walnut blight are primarily found in California and are under control with rare exceptions. These are very different diseases; the two canker diseases cause oozing of sap mostly from the trunk areas, whereas walnut blight causes a blight of leaves and nuts.


Could this disease have been named after Black Beard the pirate? Black chaff of wheat is well named because of the color of infected plant parts. This bacterium has been well researched and control is possible.