''Peronospora hyoscyami''
f.sp. ''tabacina'' is a
plant pathogen
Plant diseases are diseases in plants caused by pathogens (infectious organisms) and environmental conditions (physiological factors). Organisms that cause infectious disease include fungi, oomycetes, bacteria, viruses, viroids, virus-like orga ...
infecting
tobacco
Tobacco is the common name of several plants in the genus '' Nicotiana'' of the family Solanaceae, and the general term for any product prepared from the cured leaves of these plants. More than 70 species of tobacco are known, but the ...
that causes blue mold. It is an
oomycete
The Oomycetes (), or Oomycota, form a distinct phylogenetic lineage of fungus-like eukaryotic microorganisms within the Stramenopiles. They are filamentous and heterotrophic, and can reproduce both sexually and asexually. Sexual reproduction o ...
(a fungus-like organism) that is highly destructive toward seed plants. It is very prevalent in humid farming zones, like the
southeastern and
Eastern U.S.,
Canada
Canada is a country in North America. Its Provinces and territories of Canada, ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, making it the world's List of coun ...
, and countries bordering the
Caribbean
The Caribbean ( , ; ; ; ) is a region in the middle of the Americas centered around the Caribbean Sea in the Atlantic Ocean, North Atlantic Ocean, mostly overlapping with the West Indies. Bordered by North America to the north, Central America ...
. The disease was first identified in 1921 in
Florida
Florida ( ; ) is a U.S. state, state in the Southeastern United States, Southeastern region of the United States. It borders the Gulf of Mexico to the west, Alabama to the northwest, Georgia (U.S. state), Georgia to the north, the Atlantic ...
and
Georgia
Georgia most commonly refers to:
* Georgia (country), a country in the South Caucasus
* Georgia (U.S. state), a state in the southeastern United States
Georgia may also refer to:
People and fictional characters
* Georgia (name), a list of pe ...
.
[Ivors, Kelly, and Asimina Mila. "Burley and Flue-cured Tobacco: Blue Mold." North American Plant Disease Center Forecast. 16 July 2007. Web. 3 March 2010.] Ten years later the same disease was found once again in the same region of the U.S. The disease began to spread into
Virginia
Virginia, officially the Commonwealth of Virginia, is a U.S. state, state in the Southeastern United States, Southeastern and Mid-Atlantic (United States), Mid-Atlantic regions of the United States between the East Coast of the United States ...
,
Maryland
Maryland ( ) is a U.S. state, state in the Mid-Atlantic (United States), Mid-Atlantic region of the United States. It borders the states of Virginia to its south, West Virginia to its west, Pennsylvania to its north, and Delaware to its east ...
, and
North Carolina
North Carolina ( ) is a U.S. state, state in the Southeastern United States, Southeastern region of the United States. It is bordered by Virginia to the north, the Atlantic Ocean to the east, South Carolina to the south, Georgia (U.S. stat ...
. A few years later, the disease reached
Kentucky
Kentucky (, ), officially the Commonwealth of Kentucky, is a landlocked U.S. state, state in the Southeastern United States, Southeastern region of the United States. It borders Illinois, Indiana, and Ohio to the north, West Virginia to the ...
and
Tennessee
Tennessee (, ), officially the State of Tennessee, is a landlocked U.S. state, state in the Southeastern United States, Southeastern region of the United States. It borders Kentucky to the north, Virginia to the northeast, North Carolina t ...
. In 1960, a blue mold epidemic spread in approximately eleven countries. There was approximately twenty five million dollars in losses which is nearly thirty percent of tobacco plants at the time.
Each year, ''
Peronospora hyoscyami'' is introduced as blue mold as windblown
spore
In biology, a spore is a unit of sexual reproduction, sexual (in fungi) or asexual reproduction that may be adapted for biological dispersal, dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores fo ...
s from outside the region by infected transplants.
Symptoms

Tobacco plants with an upright position of small patches of the seed bed are typically early signs of blue mold. These leaves are usually in diameter, and normally spotted with yellow, gray, or a bluish mold in the lower surface. Younger plants that are up to four weeks old and are affected by the disease are easily killed by the oomycete. Older plants become deformed and dark, indicating that parts of the plant are dead. In more severe cases the stem and root become infected and turn to a brownish color. The observation of downy mold in the lower surface of the leaf is the most reliable symptom of blue mold. Downy mold carries thousands of small reproductive units called
conidia
A conidium ( ; : conidia), sometimes termed an asexual chlamydospore or chlamydoconidium (: chlamydoconidia), is an asexual, non- motile spore of a fungus. The word ''conidium'' comes from the Ancient Greek word for dust, ('). They are also ...
If conidia forms at different spots in the seedbed an outbreak is more than likely to occur. The symptoms usually take place before it's obvious. The symptoms evolve rapidly and are typically noticeable over night.
The oomycete frequently takes place when the
vein
Veins () are blood vessels in the circulatory system of humans and most other animals that carry blood towards the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are those of the pulmonary and feta ...
of the leaves reaches
vascular tissue
Vascular tissue is a complex transporting tissue, formed of more than one cell type, found in vascular plants. The primary components of vascular tissue are the xylem and phloem. These two tissues transport fluid and nutrients internally. T ...
of the
stem
Stem or STEM most commonly refers to:
* Plant stem, a structural axis of a vascular plant
* Stem group
* Science, technology, engineering, and mathematics
Stem or STEM can also refer to:
Language and writing
* Word stem, part of a word respon ...
, causing weakness and
lodging
Lodging refers to the use of a short-term dwelling, usually by renting the living space or sometimes through some other arrangement. People who travel and stay away from home for more than a day need lodging for sleep, rest, food, safety, shel ...
. This infection is called "systemic infection". The infection causes a stunt in growth and the leaves become narrow and short. The vascular system of the plants usually turns brown in color and the weakened stalk usually causing the stalk to fall over.
The mould is highly weather sensitive. During the time when the weather is cool, wet, or overcast the disease can evolve in a greenhouse or field. The disease spreads rapidly because of the pathogen. The rate of continental spread is based on the potential for high levels of
inoculum and effective wind spores. When the weather is clear, dry, and hot the disease usually stops spreading and more than likely stops all together.
Spreading the disease
A major way blue mold spreads is by the distribution of infected transplants. Even though they may appear to be healthy, they can still harbor the infection. Farmers generally buy transplants from other growers, and take risks of possibly buying diseased plants. If these plants are diseased they are risking their other plants' getting blue mold.
A plant may remain symptom-free for five to seven days, before the first appearance of disease. When weather conditions are reasonable, a second set of spores evolves in seven to ten days. If these plants are not chemically treated, the cycle may be repeated several times during the growing season. This can cause a huge epidemic causing many plants to be infected. Blue mold has been found to appear three times each week from March to August.
Host-pathogen interaction
After the germination of conidia, the initial penetration of the host depends on the formation of
appressoria near the
stoma
In botany, a stoma (: stomata, from Greek language, Greek ''στόμα'', "mouth"), also called a stomate (: stomates), is a pore found in the Epidermis (botany), epidermis of leaves, stems, and other organs, that controls the rate of gas exc ...
ta. This is largely due to a response of ''P. hyoscyami'' f.sp. ''tabacina'' to topographical cues of the host such as stomatal openings. Upon entry of an infection peg through a stomatal opening,
hypha
A hypha (; ) is a long, branching, filamentous structure of a fungus, oomycete, or actinobacterium. In most fungi, hyphae are the main mode of vegetative growth, and are collectively called a mycelium.
Structure
A hypha consists of one o ...
e grow and develop between plant cells. When hyphae reach a
photosynthetic
Photosynthesis ( ) is a Biological system, system of biological processes by which Photoautotrophism, photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical ener ...
mesophyll cell a peg penetrates the cell and establishes a
haustorium
In botany and mycology, a haustorium (plural haustoria) is a rootlike structure that grows into or around another structure to absorb water or nutrients. For example, in mistletoe or members of the broomrape family, the structure penetrates th ...
which the oomycete uses to absorb the necessary nutrients for its own growth.
[Borras-Hidalgo, O., Thomma, B.P.H.J., Silva, Y., Chacon, O., and Pujol, M. (2010). Tobacco blue mould disease caused by ''Peronospora hyoscyami'' f. sp. ''tabacina''. Molecular Plant Pathology 11, 13–18.] Through the establishment of haustoria the pathogen develops an intimate relationship with the host. In doing so ''P. hyoscyami'' f.sp. ''tabacina'' efficiently redirects the nutrients of the host and delivers
effector proteins into the host
cytoplasm
The cytoplasm describes all the material within a eukaryotic or prokaryotic cell, enclosed by the cell membrane, including the organelles and excluding the nucleus in eukaryotic cells. The material inside the nucleus of a eukaryotic cell a ...
that suppress the host defense response.
[
]
Tobacco defense to ''P. hyoscyami'' f.sp. ''tabacina''
The activation of systemic acquired resistance
Systemic acquired resistance (SAR) is a "whole-plant" resistance response that occurs following an earlier localized exposure to a pathogen. SAR is analogous to the innate immune system found in animals, and although there are many shared aspects b ...
(SAR) has shown to be an effective management strategy for ''P. hyoscyami'' f.sp. ''tabacina''. It has been documented that an accumulation of pathogenesis-related proteins (PR proteins) and increased activities of peroxidase
Peroxidases or peroxide reductases ( EC numberbr>1.11.1.x are a large group of enzymes which play a role in various biological processes. They are named after the fact that they commonly break up peroxides, and should not be confused with other ...
, β-1, 3-glucanase, and chitinase
Chitinases (, chitodextrinase, 1,4-β-poly-N-acetylglucosaminidase, poly-β-glucosaminidase, β-1,4-poly-N-acetyl glucosamidinase, poly ,4-(N-acetyl-β-D-glucosaminide)glycanohydrolase, (1→4)-2-acetamido-2-deoxy-β-D-glucan glycanohydrola ...
s have been shown to be associated with induced resistance in tobacco to ''P. hyoscyami'' f.sp. ''tabacina''.[Ye, X.S., Pan, S.Q., and Kuc, J. (1990). Association of pathogenesis-related proteins and activities of peroxidase, beta-1,3-glucanase and chitinase with systemic induced resistance to blue mould of tobacco but not to systemic tobacco mosaic virus. Physiological and Molecular Plant Pathology 36, 523–531.] In addition, PR proteins have been shown to have antifungal
An antifungal medication, also known as an antimycotic medication, is a pharmaceutical fungicide or fungistatic used to treat and prevent mycosis such as athlete's foot, ringworm, candidiasis (thrush), serious systemic infections such as ...
activities ''in vitro
''In vitro'' (meaning ''in glass'', or ''in the glass'') Research, studies are performed with Cell (biology), cells or biological molecules outside their normal biological context. Colloquially called "test-tube experiments", these studies in ...
'' and thus are thought to be a key component in the resistance of tobacco to blue mold.[ Of the tobacco defense responses to ''P. hyoscyami'' f.sp. ''tabacina'' SAR has been characterized as the best activated response to blue mold.
In addition to SAR, tobacco plants have been found to secrete β–ionone and T-phylloplanin which both inhibit the ]sporulation
In biology, a spore is a unit of sexual (in fungi) or asexual reproduction that may be adapted for dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores form part of the life cycles of many plant ...
and growth of ''P. hyoscyami'' f.sp. ''tabacin''a.[ T-phylloplanin proteins are secreted by tall glandular ]trichome
Trichomes (; ) are fine outgrowths or appendages on plants, algae, lichens, and certain protists. They are of diverse structure and function. Examples are hairs, glandular hairs, scales, and papillae. A covering of any kind of hair on a plant ...
s on the aerial surface of tobacco leaves and may be a novel leaf surface defense molecule in tobacco.
Prevention
The disease is getting harder to manage and is becoming a problem.[Gayed, S.K. "Blue Mold of Tobacco." Ministry of Agriculture Food and Rural Affairs. Ontario, Aug. 2009. Web. 4 July 2010.] The way to prevent the disease is to manage crops and protect plants with fungicide
Fungicides are pesticides used to kill parasitic fungi or their spores. Fungi can cause serious damage in agriculture, resulting in losses of yield and quality. Fungicides are used both in agriculture and to fight fungal infections in animals, ...
s when they are most vulnerable while taking control of the crops early on. Making the environment less favorable for the pathogen to survive and protect the tobacco will prevent its growth and spread and it is imperative to always keep the pathogen out of tobacco and the area for as long as possible. Farmers are also always able to grow their own plants rather than purchase them from others which should reduce the chance of planting diseased plants.
References
External links
Index Fungorum
USDA ARS Fungal Database
{{Taxonbar, from=Q1297964
Water mould plant pathogens and diseases
Tobacco diseases
Peronosporales
Species described in 1964
Forma specialis taxa