Q.51 Which of the following plant diseases is/are caused by bacteria?
(A) Angular leaf spot of cotton
(B) Citrus canker
(C) Apple scab
(D) Leaf curl of papaya
Angular Leaf Spot of Cotton, Citrus Canker, Apple Scab, and Leaf Curl of Papaya: Bacterial Plant Diseases Explained
Angular leaf spot of cotton and citrus canker represent key bacterial plant diseases, while the others are caused by fungi or viruses. This detailed analysis covers the causal agents for each option in the multiple-choice question, confirming the correct answers as (A) and (B).
Option Analysis
Angular leaf spot of cotton (A) causes water-soaked angular lesions on leaves due to the bacterium Xanthomonas citri pv. malvacearum, leading to yield losses of 5-27% under warm, humid conditions. The pathogen spreads via seed, rain, and wounds, with veins restricting lesion shape.
Citrus canker (B) produces corky lesions on leaves, stems, and fruit from Xanthomonas citri pv. citri, entering through stomata or injuries and causing defoliation.
Apple scab (C) results from the fungus Venturia inaequalis, forming olive-green to black lesions via ascospores, not bacteria.
Leaf curl of papaya (D) stems from Papaya leaf curl virus (PaLCuV), transmitted by whiteflies, causing leaf distortion without bacterial involvement.
Correct Answer
(A) and (B) are bacterial diseases.
Bacterial plant diseases such as angular leaf spot of cotton and citrus canker pose major threats to crops, driven by Xanthomonas species in humid environments. Understanding their causes aids CSIR NET life sciences students in plant pathology.
Causal Agents
-
Angular leaf spot of cotton arises from Xanthomonas citri pv. malvacearum, a seed-borne bacterium thriving at 86-97°F and >85% humidity.
-
Citrus canker stems from Xanthomonas citri pv. citri, producing erumpent lesions via apoplastic invasion.
Symptoms and Impact
Angular leaf spot shows angular, water-soaked spots with halos, progressing to black streaks and boll rot, cutting yields by up to 27%. Citrus canker features chlorotic halos around necrotic spots, leading to fruit drop.
Management Strategies
Use resistant varieties, acid-delinted seeds, and avoid overhead irrigation for angular leaf spot. For citrus canker, apply copper bactericides and remove infected parts. Crop rotation and sanitation curb both bacterial plant diseases.
