Blue Mold of Tobacco

Agdex#:	181/632
Publication Date:	09/83
Order#:	83-068
Last Reviewed:	08/09
History:	(Revision of Factsheet "Blue Mold of Tobacco", March 1980)
Written by:	S.K. Gayed - Agriculture Canada

Table of Contents

1. History of Blue Mold in Canada
2. Symptoms
3. Life Cycle
4. Factors Influencing the Severity of Blue Mold
5. Control

Downy mildew of tobacco commonly known as blue mold, is caused by the fungus Peronospora tabacina Adam. Blue mold was first reported about the end of the 19th century on native tobacco in Australia and Argentina. In the United States the disease was first seen in tobacco seedbeds in Georgia in 1921. In 1957 blue mold was first reported in Cuba. In 1960 a blue mold epidemic spread on tobacco in about 11 countries in Central Europe and losses were estimated at $25 million. Two years later, the disease invaded the tobacco growing areas in the Mediterranean and the Near East countries and caused severe losses. The first report of blue mold in Mexico was in 1964.

History of Blue Mold in Canada

Blue mold was first recorded in Ontario in 1938 in the old tobacco belt in Essex and Kent Counties. In 1945 and 1946 blue mold was severe in the seedbeds and caused a serious shortage of plants in the old and the new tobacco belts. Between 1948 and 1950 blue mold was mild and sporadic in tobacco seedbeds but losses were recorded in some fields in the new belt. The disease caused only negligible losses between 1951 and 1966 and there were no reports of blue mold in Ontario between 1967 and 1978.

In the 1979 season, blue mold was epidemic in the main tobacco area and total crop losses were estimated at 30% amounting to $90 million. Occurrences of the disease were observed late in the seasons of 1980, 1981 and 1983 but crop losses were negligible. In 1980 blue mold was found for the first time in the Port Hope area east of Toronto. Blue mold has not been recorded in the province of Quebec or the Maritimes.

Symptoms

In the Seedbed

Tobacco seedlings with erect leaves in small patches in the seedbed are usually the sign of early blue mold infection. Seedlings with leaves between 2 and 4 cm in diameter show clear round yellow spots on the upper surface with corresponding gray or bluish mold in the lower surface (Figure 1); at this stage some of the leaves are usually cupped. Young seedlings, up to 4-weeks old, are very susceptible to blue mold and are easily killed by the fungus. Leaves of older seedlings are puckered and deformed and dark, dead areas may develop. Diseased seedlings are stunted, and in severe cases the stem and
root become infected and turn brown in color.

The presence of the downy mold on the undersurface of the leaf is the most reliable symptom of blue mold. This downy mold carries thousands of tiny reproductive units called "conidia". If sufficient conidia form at different spots in the seedbed an outbreak is likely to occur and all seedlings in the greenhouse become infected overnight.

In the Field

Infection with blue mold in the field usually starts on the lower leaves. Yellowish round spots develop on the upper surface of the leaf with corresponding purplish to grayish mold on the lower surface. Under severe conditions the spots expand forming dark areas (Figure 2). Often the fungus penetrates the midrib and/or the veins of the leaves and reaches the vascular tissue of the stem causing wilting and lodging. Such infection of the vascular tissue of the plant is known as "systemic infection".
Systemic infection of young plants causes severe stunting and wilting and leaves become narrow and short with clear mottling. The vascular tissue of such plants turns brown and the weakened stalk may cause the plants to topple over.

Roots of tobacco plants also may be infected with blue mold.

Figure 1. Tobacco seedlings infected with blue mold. Note the yellow spots on the upper surface and the purplish gray mold on the lower surface of the leaf.

Life Cycle

Infection is usually caused by the air-borne, tiny, thin-walled conidia which germinate in a film of water on the leaf surface forming germ tubes that penetrate the leaf and branch between the leaf cells. Some of these branches enter the leaf cells from which they derive their nutrition.

Figure 2. A severe blue mold infection in the field during the 1979 epidemic.

Under cool and humid conditions, branches of the tungus emerge from the lower surface of the leaf and produce large numbers of conidia. Under favorable conditions the fungus produces as many as one million conidia per square centimetre of infected leaf surface. These are generally produced at night, become mature at dawn and are disseminated during the morning. They are carried by wind to neighboring plants or blown hundreds of kilometers to descend by rain and start new infections. This asexual cycle (spore production, infection, to more spore production) takes about 10 days and may be repeated several times during the growing season. At a later stage, sexual reproduction may occur to produce spores which represent the resting or overwintering stage of the fungus.

Factors Influencing the Severity of Blue Mold

There are 3 main factors that influence the severity of blue mold:

1. Presence of active fungus. High populations of conidia in the air increase the chance of blue mold severity. Different strains of blue mold fungus are reported in Australia and Europe. Tobacco varieties resistant to one strain may be susceptible to another. In the 1979 epidemic all tobacco varieties grown in Ontario were susceptible to blue mold.
2. Presence of susceptible tobacco. Younger seedlings are more susceptible to blue mold than older seedlings. Similarly leaves of lateral branches (suckers) are more susceptible than the harvestable leaves. Mature tobacco plants are more tolerant to the disease than younger plants.
3. Weather conditions. Saturated conditions are essential for the germination of conidia and for the sporulation of the fungus. Long cloudy periods prolong the activity of conidia. A combination of all the favorable factors results in an epidemic and the lack of some of these factors results only in slight, moderate or sporadic blue mold infections.

Control

Follow accurately the chemical control recommendations regarding the rate and timing of fungicide application in the seedbed and in the field. These recommendations are based on protecting tobacco plants against blue mold. Chemical application after the appearance of the disease may not stop its spread.

Watch carefully for blue mold symptoms in the seedbed and in the field particularly when there is a warning for blue mold incidence. If you notice any sign of the disease contact the tobacco specialists for advice. Avoid excess humidity in the greenhouse, particularly under cool, cloudy weather conditions, by proper ventilation and avoiding watering seedbeds in the afternoon.

Avoid field irrigation when a blue mold warning is in effect.