Plant Disease Basics
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Direct answer
A plant disease is the outcome of a virulent pathogen meeting a susceptible host under environmental conditions that favour the pathogen — fungi, bacteria, viruses, viroids and nematodes being the five groups to memorise. Fungi cause the greatest number of plant diseases, including rusts (Puccinia on wheat) and the white blister of crucifers (Albugo on mustard). Bacteria such as Agrobacterium tumefaciens produce crown gall, the disease biotechnology turned into a gene-transfer tool. Viruses cause mosaic and leaf-curl patterns, tobacco mosaic virus being the type example, while viroids — small naked infectious RNA molecules discovered by T. O. Diener in 1971 — cause potato spindle tuber disease. Symptom names such as mosaic, wilt, blight, canker, rot, rust and gall let you classify a disease from its photograph even before naming the pathogen.
What you must remember
- Pathogen ranking: fungi cause the majority of known plant diseases; bacteria, viruses, viroids and nematodes follow — a standard ordering in exam statements.
- Fungal examples: Puccinia (black/stem rust of wheat) and Ustilago (smut of wheat) belong to basidiomycetes; Albugo, a phycomycete, is the parasite producing white rust on mustard.
- Bacterial example: Agrobacterium tumefaciens causes crown gall by transferring T-DNA from its Ti plasmid into the plant genome — the same mechanism biotechnologists disarm to carry genes into plants.
- Viral example: tobacco mosaic virus, a rod-shaped virus with single-stranded RNA, produces mottled mosaic leaves; virus diseases spread by insects such as whiteflies and aphids.
- Viroid: potato spindle tuber disease, caused by a viroid — free RNA without a protein coat — discovered by T. O. Diener in 1971.
- Nematode example (classic): root-knot of vegetables caused by Meloidogyne, producing characteristic galls on roots — commonly cited in applied-botany questions.
- Symptom vocabulary: mosaic and leaf curl (viral), wilt and blight (often fungal or bacterial), canker and gall (bacterial), rust pustules (fungal); every symptom name points to a likely pathogen group.
- Syllabus honesty: detailed crop-disease tables came from the chapter on strategies for food production, which the rationalised NEET syllabus has dropped; the surviving NCERT anchors are the classification chapter (TMV, viroids, Albugo, rusts) and the biotechnology chapter (Agrobacterium).
From crown gall to gene gun
Trace the single best-studied plant disease. Agrobacterium tumefaciens senses a wound on a dicot stem, enters, and transfers a piece of its Ti plasmid — T-DNA — into the plant cell's nucleus. The T-DNA integrates into plant DNA and forces the cell to make plant growth hormones (producing the uncontrolled mass of a gall) and opines the bacterium feeds on. It is, in effect, natural genetic engineering: a bacterium that has been inserting genes into plants for millennia.
Plant biotechnologists reversed the exploitation. Delete the gall-causing genes from T-DNA, keep its borders and transfer machinery, insert any gene of interest — Bt toxin genes, Golden Rice genes — and Agrobacterium becomes the delivery vehicle of choice for dicot transformation. This is why one chapter's pathogen is another chapter's vector, and why a question about crown gall can appear with equal justice in a disease item or a biotechnology item.
How the exam frames it
Since the food-production chapter left the syllabus, disease questions lean on what survives: the viroid question (Diener 1971, potato spindle tuber, no protein coat) recurs because it sits inside the classification chapter, and TMV appears there too as the classic virus. The predictable trap is distinguishing viroid from virus — the viroid is naked RNA, while TMV carries a protein capsid — and students who blur them lose a guaranteed mark. Agrobacterium items now arrive from the biotechnology side, asking not about the gall but about the Ti plasmid as vector. Older disease-matching tables (late blight, citrus canker) still circulate in mock papers, so recognise them, but spend your revision time on the NCERT-anchored four: TMV, the viroid, Albugo and Agrobacterium.
Frequently asked questions
Which pathogen group causes most plant diseases?
Fungi, which is why rusts, smuts, blights and mildews dominate the disease list.
What is a viroid, and which disease does it cause?
A viroid is free infectious RNA lacking a protein coat; it causes potato spindle tuber disease, discovered by T. O. Diener in 1971.
Why is Agrobacterium tumefaciens important to biotechnology?
Its Ti plasmid naturally transfers T-DNA into plant genomes, so a disarmed version serves as a vector for introducing genes into plants.
What distinguishes tobacco mosaic virus from a viroid?
TMV has single-stranded RNA protected by a protein coat, whereas a viroid is naked RNA with no capsid.
What symptom names suggest a viral plant disease?
Mosaic mottling and leaf curling, typically accompanied by stunted growth.