Q.51 Which one of the following agents causes the necrotic ring spot disease in stone
fruits?
(A) Fungi
(B) Bacteria
(C) Virus
(D) Nematodes

The necrotic ring spot disease in stone fruits is caused by a virus, specifically Prunus necrotic ringspot virus (PNRSV).​
So, the correct answer is: (C) Virus.​

Introduction

Necrotic ring spot disease in stone fruits is a major viral disorder that reduces yield and fruit quality in crops like peach, plum, cherry, and apricot. For competitive exams, it is crucial to identify that this disease is caused by a virus, not by fungi, bacteria, or nematodes.​

Correct option: Virus (C)

• Necrotic ring spot disease of stone fruits is caused by Prunus necrotic ringspot virus (PNRSV), an RNA plant virus belonging to the genus Ilarvirus, family Bromoviridae.​

• PNRSV infects many Prunus species (peach, apricot, cherry, plum, almond) and produces chlorotic and necrotic rings, spots, and shot-hole symptoms on leaves, along with reduced yield and poor fruit quality.​

Key points for exams

• Causal organism: Prunus necrotic ringspot virus (PNRSV).​

• Disease group: Viral disease of stone fruits.​

• Transmission: Mainly through infected propagation material, pollen, and seeds.​

Why fungi (A) is incorrect

• Fungal diseases in stone fruits typically cause leaf spots, blights, rusts, or fruit rots and are associated with fungal structures like mycelium, spores, and fruiting bodies.​

• Classic necrotic ring spot of Prunus is specifically linked to PNRSV, not to any fungal pathogen, and is classified globally as a viral disease of stone fruits.​

Why bacteria (B) is incorrect

• Bacterial diseases in stone fruits, such as bacterial canker and bacterial spot, cause cankers, water-soaked lesions, gummosis, and angular leaf spots, typically associated with bacterial genera like Pseudomonas or Xanthomonas.​

• Necrotic ring spot disease does not show the typical bacterial ooze or canker pattern; instead, it presents systemic viral symptoms and is consistently associated with PNRSV infection, not any bacterial agent.​

Why nematodes (D) is incorrect

• Nematodes usually cause root galls, root lesions, stubby roots, or general decline by feeding on roots, and some nematodes can vector certain plant viruses.​

• In necrotic ring spot disease of stone fruits, nematodes are not the causal agent; the primary cause is PNRSV, and spread occurs mainly via infected propagative material, pollen, and seed rather than nematode transmission.​

Quick exam recap

• Disease: Necrotic ring spot disease in stone fruits.​

• Causal agent type: Virus → Option (C).​

• Specific virus: Prunus necrotic ringspot virus (PNRSV), genus Ilarvirus, family Bromoviridae.​