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Disease Cycle and Epidemiology

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In the spring, hyphal growth resumes from infected tissues and germinating sclerotia (Figure 22). When hyphae come into contact with susceptible crown, root, bulb, fruit or leaf tissues, direct penetration occurs, but wounds facilitate infection (Figure 18).


Figure 22	Figure 18

Infection of lower stems, roots, bulbs, fruits, and leaves may occur if tissue is susceptible and if temperature, humidity and other environmental factors are favorable. Hyphae may be intracellular or intercellular. The fungus produces oxalic acid, polygalacturonase and cellulase, all of which act to cause separation and death of cells. Within 2-4 days after infection, symptoms of soft rot are usually apparent (Figures 1, 2). When the lower trunk or stem is girdled by the soft rot canker, foliage wilt, branch dieback, and complete plant death often result (Figure 4).


Figure 1	Figure 2	Figure 4

Secondary cycles occur and disease spreads as hyphae contact new healthy susceptible tissues when warm to hot temperatures and humid conditions prevail during the growing season. Also, basidiospore production may contribute to secondary cycles (Figure 21). Once basidiospores make contact with the plant surface, spores swell and produce 1-3 germination tubes. Each germ tube may produce appressoria (swollen cushion-like structures) from which a penetration peg is produced.

Figure 21

The fungus overwinters as sclerotia and mycelium in infected plants and plant debris and sometimes as developing hymenial layers. Most sclerotia are produced at or near the soil surface and survive longer in well-drained soil (Figures 8, 9, 10, 14).


Figure 8	Figure 9

Figure 10	Figure 14

Epidemiology
Southern blight is a problem in warm and wet tropical and subtropical areas. The pathogen survives as mycelium on dead organic material when living susceptible plants are not present. It also survives as sclerotia in soil. Research has shown that fungal infection is enhanced when dead organic materials are available around susceptible plants.

Temperature and moisture are very important factors in the spread and development of this pathogen. Hyphal growth occurs over a temperature range of 8-40ºC / 46-104º F, but optimal growth and sclerotia production occurs between 27-35ºC / 81-95ºF. In addition to temperature effects, hyphal growth and sclerotia germination require a water-saturated soil. High humidity also favors fungal development. At 27ºC / 81ºF on potato dextrose agar, the hyphal growth rate of S. rolfsii has been observed to be 0.8-0.9 mm per hour. Sclerotia form after 5-7 days (Figure 23). Host penetration and infection will proceed optimally at 27-30ºC / 81-86ºF, provided that moisture and high humidity are present.

Figure 23

Current season spread of S. rolfsii within a planting occurs by mycelial growth from infected plants, plant debris, or sclerotia. Long distance spread occurs as a result of movement of infected plant material or infested soil. Studies have shown that sclerotia may pass through the digestive tract of cattle or sheep and still be viable.

S. rolfsii survives adverse conditions as sclerotia or as mycelium in diseased plants or plant debris. Sclerotia formation is favored by temperature of 30ºC / 86ºF, ample moisture, soil pH below 7, and well-aerated, light soil. Sclerotia survive best when present at or near the soil surface in well-drained soil. Research has demonstrated that no sclerotia survived when buried 15 cm (6 in.) deep for 45 days. Mycelium does not usually survive below freezing temperatures, but sclerotia are known to survive in locations where below freezing temperatures have occurred.

S. rolfsii has been reported as unusual occurrences in New York, Washington, Oregon, Indiana, Illinois, and Siberia. The fact that this fungus is usually a problem in warm climate regions indicates that sclerotia must not be tolerant of severely cold temperatures.

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