Q.42 Which of the following fungi is an example of obligate biotrophic plant pathogen?

(A) Alternaria brassicicola

(B) Botrytis cinerea

(C) Puccinia triticina

(D) Sclerotinia sclerotiorum

Puccinia triticina (C) is the obligate biotrophic plant pathogen among the options.​

Obligate biotrophs require living host cells for nutrition and cannot survive on dead tissue or culture media. They form haustoria to extract nutrients without killing host cells immediately, unlike necrotrophs or hemibiotrophs.​

Option Analysis

Alternaria brassicicola (A): This ascomycete causes black spot on Brassica crops as a necrotroph, killing cells with toxins before colonization.​

Botrytis cinerea (B): Known for gray mold, it acts as a necrotroph or hemibiotroph with a brief biotrophic phase before inducing necrosis via toxins and enzymes.​

Puccinia triticina (C): This rust fungus causes wheat leaf rust and is an obligate biotroph, relying solely on living wheat cells and forming haustoria; it cannot grow in culture.​

Sclerotinia sclerotiorum (D): Causes white mold as a necrotroph or hemibiotroph, starting with a biotrophic-like phase via oxalic acid before killing tissue.​

Introduction to Obligate Biotrophic Plant Pathogens

Obligate biotrophic plant pathogens like Puccinia triticina depend entirely on living host cells for nutrients, forming specialized haustoria without killing tissue prematurely. This contrasts with necrotrophs that thrive on dead cells. Understanding these lifestyles aids in exams like CSIR NET Life Sciences.​

Detailed Option Breakdown

• Alternaria brassicicola: Necrotrophic fungus producing toxins for Brassica black spot; kills hosts first.​

• Botrytis cinerea: Hemibiotrophic gray mold pathogen; short biotrophic phase shifts to necrotrophy.​

• Puccinia triticina: Obligate biotrophic rust on wheat leaves; cannot culture axenically, uses haustoria.​

• Sclerotinia sclerotiorum: Hemibiotrophic white mold; oxalic acid enables early living-cell phase before necrosis.​

Why Puccinia triticina Fits Perfectly

Puccinia triticina exemplifies obligate biotrophy in rust fungi (Pucciniaceae), causing global wheat losses via urediniospores and haustorial feeding. Unlike others, no necrotrophic shift occurs.​

Exam Relevance for CSIR NET

This distinction tests fungal pathogenesis knowledge; rusts like Puccinia are model biotrophs. Focus on lifestyles for competitive biology prep.​