Prepared by Damon L. Smith

A class project for PP728

Sclerotinia minor Jagger.

Introduction and importance

Sclerotinia minor, the fungus that causes Sclerotinia blight, white mold and watery soft rots of vegetables, or lettuce drop, can infect host plants from 21 families, 66 genera, and 94 species. The hosts that have been documented are from the branch Angiospermae. Most hosts of Sclerotinia minor are in the class Dicotyledonae, but there are two documented occurrences of S. minor on hosts belonging to the class Monocotyledonae (12). Sclerotinia minor occurs worldwide but especially in cool, moist regions. Sclerotinia minor causes watery soft rots, cottony soft rots, drop, and other diseases of many important flower and vegetable crops (2). Some common plants that are susceptible to S. minor include chicory, caraway, baby’s breath, carnation, and other popular ornamentals. Economically important crops that S. minor is a pathogen of include soybean, sunflower, common bean, cucumber, lettuce, spinach, cabbage, sweet potato, irish potato, pepper, tomato, peanut, and many others (12).

There has been a large amount of research completed on the various crop patho-systems that involve S. minor. This is especially true for S. minor occurring on peanut, Arachis hypogea L. Sclerotinia minor was first identified as a disease on peanut in Australia in 1948. By 1960 reports of S. minor causing root and pod rot on peanut were being reported in Argentina. The first reports of S. minor in the United States were made in Virginia 1971 and then in North Carolina in 1972 (17). Since its inception in the United States the fungal disease has become widespread, having moved into Oklahoma and Texas as well as becoming severe in Virginia and North Carolina. Sclerotinia minor is more damaging than many soilborne pathogens because it has the ability to spread rapidly within the peanut canopy, shows great persistence in the soil, as well as a high level of susceptibility in many hosts. This particular pathogen has caused yield losses of near 80 percent in some areas (17).

    Kingdom - Fungi (Mycetae)
        Phylum - Ascomycota
            Class - Discomycete
                Order - Heliotiales
                    Family - Sclerotiniaceae
                        Genus - Sclerotinia
                            Species - Sclerotinia minor

S. minor, the Causal agent of Sclerotinia blight on peanut. Picture courtesy of NCSU Plant Pathology Extension (14).

Causal agent and signs
The mycelium of S. minor is white and fluffy.  The sclerotia (survival structures) have a black outer rind, are irregularly shaped, and are approximately (0.5-3.0mm) in size.  Apothecia are rarely seen in the natural environment, but may occur during February and March or mid-fall on the soil surface.  The apothecia are pale orange to white and have concave or flat tops. If apothecia do form, they may be 6 mm or more in diameter and contain asci with ascospores (sexual spores) that measure 8-17 x 5-7 um (3).  The main mode of infection of S. minor is through eruptive germination of the sclerotia, giving rise to mycelium that infects the plant tissue (17).

Epidemiology on Peanut
Peanuts become infected with S. minor in most cases by mycelium from germinating sclerotia.  Favorable conditions for germination and infection include temperatures of 17-21 C and relative humidities of greater than 95 percent (3).  In addition, a soil pH of 6.5 is considered desirable for the germination of sclerotia in the field (17).  In addition, presence of volatile materials from moist undecomposed plant tissue has been shown to initiate sclerotial germination in closely related fungi such as Sclerotium rolfsii. Substances include alcohols such as methanol, aldehydes, esters, and halogenated hydrocarbons (5).  Presence of volatiles has also been shown to be related to sclerotial germination by S. minor (17). Under optimum conditions peanut leaves, stems, and pegs that are in contact with the ground can be infected by the growing mycelium of S. minor.  Interestingly, unlike other species of Sclerotinia, S. minor does not need a food base of dead or senescing tissue to infect.  It has the capability to infect stems and branches in contact with the soil with no food bases (17).  During severe infections of peanut crops, the top 2.54 cm of soil can be heavily infested with sclerotia.  At the end of the growing season, sclerotial densities of 50 sclerotia per 100 g of soil are possible (17).
Mechanically injured peanut tissue is very susceptible to colonization by S. minor.  Plants injured by tractor tires during pesticide application showed a greater incidence of disease caused by S. minor than non-injured plants (3).  In addition, yield losses correlated to plant injury were about twice as great as those yield losses in non-injured plants (17).
 
 

Disease Cycle
The mycelium of Sclerotinia minor attacks lower branches and rapidly invades succulent tissues causing cells to collapse.  The mycelium then develops a white, fluffy mass on the surface of the tissue as it invades more plant tissue.  The plant is eventually killed and sclerotia are produced in abundant numbers on the dead tissue.  Some sclerotia are shed from the plant tissue into the soil or can be retained on dead plant tissue as overwintering inoculum.  When conditions are right, the sclerotia germinate into either mycelium or apothecia (sexual stage of the fungus).  In the mycelial stage the fungus infects a new plant and the cycle starts over again.  If the sclerotia give rise to apothecia (which is a very rare occurrence in S. minor) then asci are formed with ascospores (the sexual spores).  When the ascospores mature they are discharged and land on a plant, germinate, and start the disease cycle over again (3).

Signs and Symptoms
Sclerotinia minor can cause the watery soft rot of many fruits and vegetables except onions and potatoes (2).  In a cool (18 degrees C) moist environment (greater than 95 percent humidity) watery soft lesions occur at sites of infection and the tissues are rapidly covered with white fluffy mycelium.  In peanuts, all tissues including pegs and pods also show watery lesions and can succumb to white mycelium (3).  The mycelium can penetrate into the tissue intercellularly and gain nutrients in this manner.  Eventually, yellowish-brown bleached lesions appear on stems and pegs.  The leaves become chlorotic and necrotic, and stems become girdled and die.  On the dead plant tissue, as well as within tissue such as stems, sclerotia form and the plant tissue is eventually shed to the soil where the sclerotia break off and become dormant in the soil or can remain on or in plant debris.

Mycelium and sclerotia on a bleached peanut stem.  Photo courtesy of Joyce Hollowell.	Sclerotia on peanut pods. Picture courtesy of Joyce Hollowell.

Current areas of Research
Some other research that is being conducted involves the identification of weed species that maybe found in or near peanut fields.  This is becoming a focus of research as it is believed that the weeds may act as a source of overwintering inoculum of S. minor as many weeds do for viral pathogens such as Tomato spotted wilt virus (TSWV) (1).  A weed that has been reported as a host of Sclerotinia minor is Eclipta prostrata.  Eclipta prostrata is a member of the Asteraceae that has been found in peanut fields of Oklahoma.  Eclipta has become an established weed in peanut fields, infesting as much as 4,000 ha of irrigated peanut in Oklahoma (11).  High populations of this weed in peanut fields are thought to be due to excessive rain or irrigation and cool temperatures, which also favor growth of S. minor (11).  Sclerotinia minor infection and the presence of sclerotia are abundant on many plants of E. prostrata.  In the lab, E. prostrata tissue had about four to eight viable sclerotia per 50 mm of lesion, 15 days after inoculation with S. minor.  This information and data suggests that Eclipta is a potential reservoir for S. minor in field soils in the absence of peanut plants (11).  Eclipta has the potential to be a source of primary inoculum that can infect peanut crops.
 Another weed often found in and around peanut fields is yellow nutsedge (Cyperus esculentus L.).  In September of 2000, the signs and symptoms of S. minor were observed on yellow nutsedge in Bertie County by Joyce Hollowell (9).  Diseased plants showing signs and symptoms of S. minor were taken from the Peanut Belt Research Station at Lewiston-Woodville, NC.  The fungus was isolated from the plant and grown on potato dextrose agar (PDA).  After performing Koch’s Postulates on the fungal isolate, it was determined that it was indeed S. minor (9).  This was the first time that S. minor was reported on yellow nutsedge.  Although more research needs to be conducted on this weed, it is thought that it could act as a reservoir for S. minor and a source of primary inoculum that can infect peanut.  In addition, other weeds need to be examined and their susceptibility to S. minor infection and their ability to harbor the fungus determined.  If such weeds act as alternative hosts for S. minor it will be necessary to control weed populations around peanut fields to reduce sources of primary and secondary inoculum.  Weed control around peanut fields is already a high priority to control TSWV.  This will likely become an even larger priority as more weeds are discovered that can harbor pathogens of peanuts such as S. minor.
 
 

S. minor on yellow nutsedge (Cyperus esculentus L.). Photo Courtesy of Joyce Hollowell.

Another topic of S. minor that has not been researched extensively is the interaction of plant architecture and S. minor.  Little is known about how the fungus infects peanut and what part or parts of the peanut plant are most susceptible to infection of S. minor.  In addition, there has been minimal research conducted concerning the interaction between the environment and S. minor infection.  Dow, et al(8) found that the optimum temperature of germination, mycelial growth, and infection was 20-25 C.  Ninety-five to 100% relative humidities were also favorable (8).  Imolehin, et al.(10) studied the effects of temperature and moisture on sclerotial production, germination, and infection by S. minor on lettuce tissue.  In the lab, the optimum temperature for sclerotial germination was 18C, while 12C was the optimum temperature for sclerotial formation.  Solute potentials of –7 bars was optimal for mycelial growth, while germination of sclerotia was optimal at –1/3 bars (10).  Research on soil moisture and temperature and its effects on S. minor germination and growth needs to be conducted in field studies to determine the true environmental influences on the behavior of S. minor.    Better understanding of these areas is needed in order to better understand how to control the fungus.  If more understanding and better control of the fungus are not obtained this problem may become of epidemic proportions in the near future. 

Literature Cited

NC State, A&T State University Cooperative Extension: Peanut Page

Georgia Cooperative Extension Service: Georgia Commodities

University of Illinois Extension: Sclerotinia Disease

University of Florida Cooperative Extension Service: Sclerotinia Diseases of Vegetable and Field Crops in Florida

University of California: Agriculture and Natural Resources: Sclerotinia Diseases

NC State Peanut Pathology Extension Leader

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