Plant Diseases – Fungi

Fungi are the last category of plant diseases in our year-long study of plant diseases that can affect our plants in the San Francisco Bay Area. Other categories have included bacteria, plant parasites, water molds, nematodes, and viruses. Our resources have included information from my plant diseases course at Merritt College in Fall 2012, Essential Plant Pathology, and Five Kingdoms. As with other categories, many fungi are beneficial for the ecosystem, but others can cause disease or damage.

Fungi are incapable of producing their own food, so seek nourishment from dead organic matter or living plants. Fungi are composed of filaments, which excrete enzymes that decompose organic matter so that the cells can absorb the nutrients. An individual filament is a hypha (hyphae), and the mass of filaments is the mycelium. Some parasitic fungi produce toxins that help them invade a host; others produce mycotoxins that are poisonous, causing illness or death to animals or humans who ingest them.

White mycelia of a wood decay fungi - great on the forest floor, on your roof - not so much.

Hypha cells contain mitochondria, nuclei, and other organelles. Nuclei are haploid (one set of chromosomes). Cell walls are made of chitin (similar to the shells of crabs and exoskeleton of insects), rather than cellulose (the main component of plant cell walls), making fungi more closely related to animals than plants. Additionally, fungi use glycogen for food storage (similar to animals), rather than starch (as plants do). The hypha may grow into survival structures (called fruiting bodies) that produce spores, such as puff balls, mushroom, and conks. Fungi reproduce by sexual and asexual spores. Sexual spores ensure genetic diversity and adaptability; asexual spores are clones that ensure survivability.

The four major phyla of true fungi are Ascomycota, Basidiomycota, Chytridiomycota, and Zygomycota. The phyla are called true fungi to distinguish them from organisms that were formerly classified as fungi, such as the water molds (or Oomycetes). The first two groups are especially important for plant diseases:

• Ascomycetes – bread and beer yeasts, morel mushrooms, and plant parasites that cause common diseases such as Chestnut blight, apple scab, leaf spots, Dutch Elm disease, post-harvest diseases, powdery mildews, and vascular wilts
• Basidiomycetes – root rots, rust fungi, smut fungi, wood decay fungi, turf grass diseases, and Sclerotium spp. blight

All parts of a plant—roots, stems, vascular system, trunks, flowers, and leaves—are susceptible to fungal diseases.