Boll rot is a plant disease that targets the reproductive structures of certain plants, causing damage and crop losses. This disease involves various pathogens, predominantly fungi and bacteria, that infect the developing fruit. It compromises crop quality and yield.
Understanding the Boll
A boll refers to a specialized, rounded seed vessel or capsule characteristic of certain plants, such as cotton and flax. It encases developing seeds and lint, the valuable fiber in plants like cotton. This capsule makes plants susceptible to boll rot, as the disease directly affects this fruiting body. Each boll contains multiple compartments, called locks, where cotton seeds and fibers grow until maturity.
Cotton: The Primary Host of Boll Rot
Cotton (Gossypium species) is the primary plant affected by boll rot, causing yield and quality reductions. Both fungal and bacterial pathogens contribute to cotton boll rot. Common fungal pathogens include Fusarium moniliforme, Colletotrichum capsici, Aspergillus flavus, Aspergillus niger, Rhizopus nigricans, Nematospora nagpuri, and Botryodiplodia sp. Fusarium species, such as Fusarium oxysporum f. sp. vasinfectum, are common causes in the U.S.
Bacterial blight, caused by Xanthomonas citri pv. malvacearum, also causes boll rot, starting as small, water-soaked lesions that blacken. Alternaria and Phytophthora species can also infect bolls, with Phytophthora boll rot causing soft, watery, black symptoms. These pathogens penetrate the boll, decaying inner tissues, seeds, and lint, which compromises fiber quality and quantity.
Recognizing Boll Rot Symptoms
Boll rot symptoms on bolls begin with small, light brown spots that enlarge and darken. These spots expand to cover the boll, turning it dark brown or black. Affected bolls may develop a soft, water-soaked appearance, indicating internal decay. As the disease progresses, bolls may fail to open or open prematurely, revealing stained and degraded fibers.
In severe cases, visible fungal growth, such as white mold or black smut-like masses, may appear on the boll surface, especially under humid conditions. Internal lint and seeds become discolored, turning yellow, brown, or black, which reduces the cotton’s market value. Infected bolls can also drop prematurely, leading to yield loss, sometimes as high as 20% in severe outbreaks.
Factors Promoting Boll Rot Development
Environmental and physiological conditions contribute to boll rot development in susceptible plants like cotton. High humidity and prolonged rainfall are primary factors, creating a moist environment for pathogens to thrive and infect bolls. Warm temperatures accelerate the growth of these fungal and bacterial organisms.
Dense canopy cover from excessive plant growth or close spacing traps moisture and reduces air circulation around bolls, exacerbating humidity. Plant injuries, such as from insect feeding or mechanical damage, provide entry points for pathogens. Excessive nitrogen application and over-irrigation can also promote dense growth, making plants more vulnerable.