The Blue Mold Disease of Tobacco
Department of Plant Pathology
N.C. State University
Raleigh, NC 27695
Blue mold of tobacco (also known as 'mildiou du tabac' in Europe), caused by Peronospora tabacina Adam, is a classical compound-interest plant disease that develops into local as well as macroscale epidemics. The fungus is highly weather-sensitive. During periods of cool, wet, and overcast weather the disease develops and spreads rapidly because of the polycyclic multiplication of the pathogen. The rapid rate of development is determined by potentially high levels of initial source inoculum, short latent period, and large numbers of effective dispersal units or spores. When the weather becomes clear, hot, and dry, the epidemic usually slows or stops completely.
Commercial tobacco is a seasonal crop in the temperate, warm and cool, humid farming zones of the Southeastern and Eastern United States, Canada, and countries bordering the Caribbean basin. Following a crop-free period (winter) each year, the tobacco in the U.S. is exposed to asexual, windborne sporanagiospores originating from inoculum sources on commercial winter tobacco and wild Nicotiana species in the tropical zones south of the 30th parallel of latitude. The fungus is not known to overwinter in the more temperate zones, so inoculum is introduced anew each year into the U.S.
Signs and Symptoms
Tobacco blue mold is fairly easy to diagnose. The symptoms vary with plant age. On seed beds of small seedlings with leaves less than 2 cm in diameter, small patches of dead or dying seedlings with erect leaves provide evidence of the disease. Among older plants with leaves up to 4 cm in diameter, blue mold is first seen as circular, yellow areas of diseased seedlings. Plants in the center of the affected area may have distinctly cupped leaves. Some of the cupped leaves exhibit a gray or bluish downy mold on the lower surface; hence the name blue mold. The upper surfaces of infected leaves can remain almost normal in appearance for 1-2 days before the plants begin to die and turn light brown. Diseased leaves can become so twisted that the lower surfaces turn upward. In such cases, the bluish color of the diseased plants becomes quite conspicuous, especially under wet conditions when sporulation is abundant.
The blue mold progresses slowly at first. After 7-10 days, however, when sufficient secondary inoculum has been produced, a general epidemic occurs and the entire seedbed becomes affected almost overnight. When the weather is cloudy and cool, the fungus can kill plants in large areas of the bed. A characteristic foul odor of rapidly decaying vegetative matter develops. When the weather turns sunny and warm, plants with few symptoms sometimes recover as the fungus stops sporulating and plants begin to put out new leaves. However, seedlings with any type of symptoms should never be transplanted to the field. Whenpurchasing seedlings from a commercial producer, make sure the plants have been certified as disease free. Use locally grown seedlings if possible. In its early stages blue mold can easily be confused with cold injury, malnutrition, or damping-off. However, the presence of the characteristic downy fungus spores on the bottom of leaves quickly identifies the disease as blue mold and distinguishes it from other problems.
Blue mold may affect plants in the field throughout the growing season. Single or groups of yellow spots (lesions) appear on the older, shaded leaves. Often the spots coalesce to form light brown, necrotic areas. Leaves become puckered and distorted, large portions disintegrate, and the entire leaf may become unusable. Under continued favorable weather conditions the fungus can destroy all leaves at any growth stage. Lesions may occur on buds, flowers, and capsules; sporangiospores occasionally form on the calyx and capsules but have not been found on the stems. Plants sometimes exhibit wilt symptoms with narrow, stunted, mottled leaves.
Systemic stem infection, resulting in partial or overall stunting of the plant, is common in some regions. Leaves may become twisted or distorted. Vascular discoloration occurs inside the stems, and lesions are visible as brown streaks. If this occurs near the base of weakened stems, the plants often lodge or snap off.
Causal Organism and Disease Cycle
The most commonly used name for the blue mold fungus is Peronospora tabacina Adam. Nicotiana species are the ONLY known hosts. The name Peronospora hyoscyami de Bary has priority, but insufficient documentation is available to accept this name for the tobacco pathogen. The blue mold fungus is in the class Oomycetes, order Peronosporales, and the family Peronosporaceae. It is an obligate parasite (biotroph) and produces both sporangiospores and oospores. The hyaline, lemon-shaped sporangiospores (15 x 25 micrometers) are borne on tree like, dichotomously branched sporangiophores, which terminate in curved, acute apices. The sporangiophores emerge through the stomates on the underside of the leaf in great numbers and vary from 400 to 750 micrometers in length. The sporangiospores are fragile and short-lived. They are sensitive to UV light, and when released, exposure to direct sunlight kills most within an hour. They seldom are found viable on leaf lesions more than 72-96 hours old. The fungus has been reported to produce as many as one million sporangiospores per square centimeter of infected leaf surface. These sporangiospores are released, float in the air like fragile balloons and are easily spread long distances by the wind.
Oospores are sometimes produced in the mesophyll of dead parts of the infected plant. Mature oospores are usually reddish brown and 20-60 micrometers in diameter; their size varies under different conditions. Strains that differ in pathogenicity, virulence, host reactions, sporulation capacity, optimal growth temperature and resistance to fungicides have been reported. The rapid appearance of new strains or races of the fungus that vary widely in pathogenicity following release of resistant varieties and/or new fungicides indicates the genetic plasticity and adaptability of this unique and dangerous plant pathogen.
Blue Mold is a local, regional, and continental problem. Inoculum produced in one zone of the North American continent can quickly be transported via the atmosphere to our production areas far distant. The fungus is not known to overwinter in the Southeastern U.S. north of 30N latitude. Tobacco is a seasonal crop in the temperate agricultural areas, and following a crop-free period each year (winter), the tobacco is exposed to windborne spores. The epidemics are usually cyclic (yearly) and progressive; once established, they advance as a more or less definable front via windborne spores. In some years and areas, however, new outbreaks appear hundreds of kilometers beyond the perceptible epidemic front as isolated, local epidemics. In such cases, long-distance transport of inoculum has been documented. It is not uncommon to experience no blue mold in a given year, or to have local areas that periodically escape the disease. The difference between continuous and discontinuous epidemic fronts is related to inoculum dispersal patterns, localized weather, density and spatial aggregation of tobacco fields within a production region, and planting schedules. The occurrence, intensity, and distribution pattern of blue mold can be greatly affected by coordinated efforts to manage the disease with chemical fungicides.
Once the airborne sporangiospores arrive during the morning hours at the leaf surface, with free water, germination and infection can occur in as little as 2-4 hr. A 5- to 7-day, symptom-free incubation period takes place before the appearance of the first visible symptoms (yellow lesions). Incubation becomes longer with less than ideal conditions and with the age of the tobacco plants. The latent period for sporulation is generally 5-7 days. Sporulation can occur on the day symptoms first appear, but it usually occurs on the following night. For sporangiospores to appear, relative humidity must exceed 95% for 3 hr and darkness must last a minimum of 1.5 hr. Maximum sporulation occurs at 15-23 C, although some sporulation occurs at temperatures of 1-2 C and 35 C. However, with increased duration of exposure to high temperatures (i.e., 35-40 C) interposed upon a 20 C incubation period, lesion development and sporulation are delayed or completely suppressed. Day temperatures in excess of 30 C for more than 6 hr have been reported to inhibit sporulation the following night. However, since 1979, thermophilic strains of P. tabacina have been documented to survive and reproduce at temperatures as high as 35 C (95 F).
Spore liberation occurs with a rise in temperature, a decrease in relative humidity, and a change in turgor pressure within the leaf. Sporangiophores of P. tabacina react to dry air by desiccating and twisting counterclockwise. As the entangled sporangiophore branches disengage, spores are mechanically ejected from the leaf surface by the resulting spring action. Solar radiation has also been implicated in spore release.
Sporangiospores are produced during the night hours. As many as one million spores can be found on a single disease lesion. Each morning the spores are released and the concentration in the air over a field can increase rapidly reaching a maximum at about 10:00 AM local time. Spores are usually released between 0800 and 1500 hr. Spore concentrations as high as 1.4 X 1011 spores have been measured in the air during a 2-hr period over 100 ha of severely infected tobacco. On days when clear conditions alternate with long overcast periods, spore release exhibits a series of peaks that correspond to the sunny periods. With the onset of rain, a short period of spore liberation occurs regardless of the wind. If a steady rain continues for several hours, the air is effectively cleansed of sporangiospores after the initial peak. The number of rain-liberated sporangiospores is usually higher during the day than at night.
Another common way to spread the blue mold fungus is by moving infected transplants. In some cases, transplants that appear healthy may actually be infected. Farmers periodically buy transplants, sometimes from distant growers, and run the risk of buying diseased plants. Only certified disease-free transplants should be sought and purchased. Sporangiospores may be disseminated physically by workers, animals, cars, and airplanes. These agents are generally considered to be of minor importance, but, considering the copious amounts of inoculum and the rapidity of modern transportation, such mechanisms have been implicated in outbreaks hundreds of kilometers from known inoculum sources.
In the warmer areas of tobacco production, tobacco plants sometimes remain alive over the winter (volunteers) and produce a new crop of shoots (suckers) in the spring. Freezing temperatures that kill only the portion of the plant above ground do not always eliminate the fungus. It can remain alive in the roots and underground dormant buds as systemic myceluim. Oospores have been suggested as another method of fungus dissemination, although this mechanism has never been documented. Seed transmission has been reported once in Australia but never confirmed elsewhere.
Forecasting the Transport of Spores
The long-range atmospheric transport of P. tabacina sporangiospores has been extensively studied in an attempt to explain the spatial and temporal patterns of blue mold epidemics. The U. S. Air Resources Laboratory's Atmospheric Transport and Dispersion Model (ATAD), and an improved version known as the Branching Atmospheric Trajectory Model (BAT), have been used to identify primary inoculum source areas and pathways of moving spore clouds. Together with real-time weather prediction systems of NOAA that forecst weather 48 hours into the future, the HY-SPLIT and MASS trajectory models offer promising forecasting capabilities. Such a North American Blue Mold Forecasting system is presently operating at North Carolina State University and can be accessed on the Internet at http://www.ces.ncsu.edu/depts/pp/bluemold/. A description of the Forecast System is covered in another section of this report.
The blue mold-tobacco continental pathosystem, operating as it does between the Caribbean, Latin America, the continental United States and Canada, provides one of the most interesting and promising case studies for using the plant disease forecasting by applying the information and tools of biology, aerobiology, and meteorology. Blue mold is a continental problem, and its successful management will require the cooperation of scientists and officials from all countries where the disease occurs. The threat of recurring epidemics each year should reinforce the concept of collective responsibility and the need for collaboration among countries faced with managing this potentially devastating disease.
Page last updated by Matthew Miller on 25 May 2005