Fungi and Plant Disease

Fungi are one of the living organisms that can cause plant disease and are the cause of about eighty-five percent of all plant diseases.

More than 100,000 species of fungi have been classified and include molds, mildews, and mushrooms. Most are beneficial or benign, with only about eight percent of fungal species causing plant diseases. Unlike plants, fungi do not have chlorophyll and cannot photosynthesize. Instead they must rely on other living things for sustenance.

A conk growing on the side of a tree trunk

A "conk" growing on a tree trunk.

Usually a microscope is needed to see and identify fungi, but they are sometimes visible with the naked eye. Examples include large colonies of target spot fungus on tomato or a conk (shelf fungus) growing on a palm tree.

Some fungi reproduce through mycelia, including Rhizoctonia spp. that produces branch-like fungal threads that can survive for long periods before germinating. However, most fungi reproduce via spores. Spores provide the fungus with a great means for dispersal. Their lightweight nature means that they can be carried for miles on the wind. Spores can be moved from one place to another by splashing water, either from rainfall or irrigation. Spores and mycelia can also travel in soil when infected soil from one area of the garden is transferred to the soil around a healthy plant. Spores can sit inactive for long periods prior to germination, which will only occur in the presence of water or high moisture levels. Once the spores have germinated, the fungus can penetrate and infect plant tissue through a plant’s cuticle, through natural openings (called stomata) in the plant’s leaves or stems, or through wounds.

Diseases like powdery mildew, cercospora leaf spot, take all root rot, and anthracnose are caused by different fungal species. However, the majority of plant diseases are caused by two main groups of fungus—those from Phylum Ascomycota and Phylum Basidiomycota.

Fungi in Ascomycota are commonly called sac fungi and include these species:

  • Diplocarpon rosae – causes black spot on roses
  • Fusarium spp. – causes fusarium wilt
  • Thielaviopsis spp. – causes canker rot and black root rot
  • Verticillum spp. – causes verticillum wilt
  • Magnaporthe grisea – causes gray leaf spot in turfgrasses
  • Colletotrichum gloeosporioides – causes anthracnose in mangoes, papayas, and other plants

Fungi in Basidiomycota are commonly called club fungi and include these species:

  • Rhizoctonia solani – causes brown patch in St. Augustinegrass
  • Phakospora pachyrhizi – causes soybean rust
  • Ganoderma zonatum – causes ganoderma butt rot in palms

Other fungal-like organisms can also cause plant disease. Myxomycetes are commonly called slime molds. Oomycetes are commonly called water molds and include Pythium spp., an organism that is not host-specific and that causes root rot and damping off. Another oomycete is Phytophthora spp. This is a host-specific organism that affects mainly dicotyledonous  plants and was the source of the potato blight that cause famine in Ireland in the 1800s. Both myxomycetes and oomycetes were once classified as fungi but are not true fungi.