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Asexual Trichoderma harzianum. This structure is called a conidiophore. Asexual T. harzianum spores form in the flask-shaped cells at the tips of its branches (Image courtesy SBML).

The sexual spores of Hyprocrea lixii are green. H. lixii is the sexual stage of T. harzianum. (Image courtesy SBML).

Researchers Find Reproductive States of Two Beneficial Fungi

In the world of fungi, asexuality--that is, reproduction without sex--is not uncommon. Unfortunately, species that clone themselves cannot crossbreed and improve desirable qualities. Changes come about only through random genetic mutations.

Now, scientists with the Agricultural Research Service and Pennsylvania State University have found a natural solution to this problem for at least two important fungal species. The scientists have discovered the sexual states, or teleomorphs, of Trichoderma harzianum and T. atroviride.

Fungi in the genus Hypocrea, often as their asexual Trichoderma states, are useful in the biological control of costly plant diseases. T. harzianum can be used to control botrytis gray mold, a pathogen that causes disease in strawberries, and it has the potential to enhance plant growth. T. harzianum has also been credited with degrading pesticides in soil and preventing mycotoxin synthesis. T. atroviride has been shown to control snow mold fungi on winter cereals and turf grasses, and certain root rot diseases that attack ornamentals.

Priscila Chaverri, a Penn State graduate student working with mycologist Gary Samuels at ARS’ Systematic Botany and Mycology Laboratory (SBML) in Beltsville, Md., has identified the sexual state of T. harzianum as Hypocrea lixii. She confirmed it with DNA sequence analysis. Although H. lixii was first described in the 19th century, no one had made the connection between it and T. harzianum until now. The journal Mycological Progress published a paper about the discovery in August.

Meanwhile, Sarah Dodd, a Penn State post-doctoral student who is also working with Samuels at SBML, has discovered the sexual state of T. atroviride. She and Samuels named the species--which is new to science--Hypocrea atroviridis and confirmed the link through molecular analysis. A paper about this discovery has been accepted for publication in the journal Mycologia.

Discovering the sexual states of two Trichoderma species suggests that more genetic variations of these fungi exist in nature than was previously assumed. Plant pathologists working to develop nonchemical means to control plant diseases may be able to use this information to find better biocontrol agents.

ARS is the U.S. Department of Agriculture’s chief scientific research agency.