Plant Pathology and Plant-Microbe Interactions

Plant disease causal organisms

A plant disease is usually identified by its causal organism, and the pathogen group determines how the disease spreads, how it is diagnosed and how it is controlled. The main groups of plant pathogens are true fungi, oomycetes (water moulds, which are not true fungi), bacteria, viruses, phytoplasmas, nematodes and a few protists. Smuts, rusts, mildews and ergots are all fungal diseases, but their fungal classes differ, so the disease name alone is not enough to identify the pathogen.

The following pairs should be memorized as disease, causal organism and host.

• Karnal bunt of wheat is caused by the smut fungus Neovossia indica, a basidiomycete.
• Ergot of rye is caused by the ascomycete Claviceps purpurea, which replaces grains with sclerotia.
• Late blight of potato is caused by the oomycete Phytophthora infestans, not by a bacterium or a true fungus.
• Powdery mildew of barley is caused by the ascomycete Blumeria graminis, an obligate biotroph.
• Leaf rust of wheat is caused by the rust fungus Puccinia recondita, a basidiomycete.
• Early blight of potato and tomato is caused by Alternaria solani, a fungus and not an oomycete.
• Blast of rice is caused by the ascomycete Magnaporthe grisea, whose asexual stage is Pyricularia oryzae.
• False smut of rice is caused by the fungus Ustilaginoidea virens, which forms smut balls on individual grains.
• Sheath blight of rice is caused by the soil-borne basidiomycete Rhizoctonia solani.
• Downy mildew of rice is caused by the oomycete Sclerophthora macrospora.
• Cereal cyst of barley and ear-cockle of wheat are caused by plant-parasitic nematodes, Heterodera avenae and Anguina tritici respectively.
• Wart of potato is caused by the chytrid fungus Synchytrium endobioticum, so it is not a nematode disease.
• Root knot of tomato is caused by root-knot nematodes of the genus Meloidogyne.
• Black rot of crucifers is caused by the bacterium Xanthomonas campestris pv. campestris.
• Tungro of rice is caused by viruses transmitted by leafhoppers, not by mycoplasmas.
• Necrotic ring spot of stone fruits is caused by Prunus necrotic ring spot virus, an ilarvirus.
• Loose smut of barley is caused by the smut fungus Ustilago nuda, and grape downy mildew is caused by the oomycete Plasmopara viticola.
• Red rot of sugarcane is caused by the fungus Colletotrichum falcatum.
• Tikka disease of groundnut is a leaf spot caused by the fungus Cercospora personata.
• White rust of mustard is caused by the oomycete Albugo candida, and clubroot of crucifers is caused by the protist Plasmodiophora brassicae.

When a question asks about the type of causal agent, remember that oomycetes such as Phytophthora and Albugo are not fungi, and that Plasmodiophora is a protist and not a fungus.

Salicylic acid and jasmonate defense signaling

Plants coordinate their immune responses mainly through two hormone-based signals, salicylic acid (SA) and jasmonic acid (JA) with its derivative jasmonoyl-isoleucine (JA-Ile). The SA pathway is generally effective against biotrophic pathogens that feed on living host cells, and its activation leads to systemic acquired resistance. The JA pathway, often working together with ethylene, provides defense against necrotrophic pathogens and herbivorous insects.

Systemic acquired resistance (SAR) is a long-lasting, broad-spectrum resistance that develops throughout the plant after an initial local infection. It depends on SA accumulation in distal tissues, and SA-responsive genes such as PR-1 are induced. NPR1 is a central regulator of SA-dependent gene expression.