Title: Image Hotspots - Target Spot Disease Cycle

Overwintering/Dormancy

The fungus survives unfavorable conditions in the soil as hyphae on debris or as small structures called sclerotia.

[Image: Hyphae of Rhizoctonia solani]
Credits: Hyphae of Rhizoctonia solani; by Arlene Mendoza-Moran; 2013; licence C

Video: Survival and Initial Inoculum Production

Credits: Target spot survival and initial inoculum production; by North Carolina State University; 2016; licence C

Initial inoculum production

The pathogen forms a thick mat of hyphae called a hymenium. The hymenium then produces structures called basidia, which give rise to small spores called basidiospores. These basidiospores are then dispersed through the air.

The hymenium on this greenhouse tobacco leaf appears as coarse white threads. Hymenia may also form on the soil surface.

[Image: Hymenium of Rhizoctonia solani on tobacco]
Credits: Hymenium of Rhizoctonia solani on tobacco; by David Shew; licence C

Video: Inoculation and Infection

Credits: Target spot inoculation and infection; by North Carolina State University; 2016; licence C

Infection

When a basidiospore contacts the leaf surface it may directly penetrate the cell and begin growing within it.

[Image: Basidiospore of Rhizoctonia solani infecting a tobacco cell]
Credits: Basidiospore infecting tobacco cell; by David Shew; licence C

Primary lesion formation

As the cells adjacent to the initial infected cell die, a pin-sized circular spot of dead tissue forms.  These spots, called primary lesions, are about 2 to 3mm in diameter and light brown or white in color.

[Image: Target spot primary lesions]
Credits: Target spot primary lesions; by Arlene Mendoza-Moran; 2016; licence C

Video: Colonization and Symptom Development

Credits: Target spot colonization and symptom development; by North Carolina State University; 2016; licence C

Lesion expansion

Primary lesions can expand to form alternating bands of light and dark brown, leading to a target-like appearance.

[Image: Target spot lesion]
Credits: Target spot lesion showing primary lesion; by David Shew; 2016; licence C

Video: Secondary Inoculum

Credits: Target spot secondary inoculum and infection; by North Carolina State University; 2016; licence C

Infected tissue falls out

The tissue in the centers of lesions eventually becomes thin and brittle.  Pieces of tissue often fall out, leaving a hole in the leaf.  These pieces land on the ground, where they may remain in the soil until the next growing season.

[Image: Target spot lesion with shot hole]
Credits: Target spot lesion with shothole; by David Shew; 2016; licence C

Credits: Target spot of tobacco disease cycle; by Arlene Mendoza-Moran; 2016; licence C