Corynespora cassiicola is a widespread fungal plant pathogen causing significant concern in global agriculture. This fungus can infect over 500 plant species across 53 families, including many economically important crops worldwide. Its presence is particularly prevalent in tropical and subtropical regions, where it thrives and causes crop losses. The pathogen exhibits high genetic diversity, which contributes to its adaptability across various hosts and environments, making it a threat to food security and agricultural economies.
Diseases and Crop Impact
Corynespora cassiicola causes several plant diseases, leading to economic impact across agricultural systems. One prominent example is target spot disease, which affects a wide range of crops, including tomatoes, cotton, and soybeans.
In tomatoes, target spot manifests as distinct lesions on leaves, initially appearing as dark brown pinpoint spots that enlarge into circular areas with light brown to gray centers and dark outer concentric rings, often surrounded by a yellow halo. These symptoms can easily be confused with other diseases like early blight or bacterial spot. The disease also causes sunken flecks on tomato fruits that develop into deeply pitted lesions, leading to post-harvest fruit rot and unmarketable shipments.
In Florida, where fresh-market tomatoes generate nearly half a billion dollars annually, target spot has become one of the most damaging foliar and fruit diseases, with significant losses. Similarly, target spot on cotton can cause premature defoliation during boll set and maturation, resulting in significant yield reductions.
Another disease caused by C. cassiicola is Corynespora leaf fall (CLF) in rubber trees (Hevea brasiliensis), a major source of natural rubber. This disease is prevalent in Asia and Africa, where it causes necrotic lesions on leaves, often described as “fishbone” or “railway track” shaped due to blackening of the veins. Repeated defoliation weakens the trees, directly impacting latex production, as rubber synthesis is closely tied to the photosynthetic activity of the leaves. CLF can lead to natural rubber production losses ranging from 20% to 45% in affected plantations, threatening the global rubber industry.
Life Cycle and Environmental Factors
The life cycle of Corynespora cassiicola involves its spread, survival, and infection of host plants. The fungus primarily exists as asexual spores, known as conidia, and vegetative mycelium in nature. These spores are the main means of dispersal and infection, travelling through the air via wind and rain, or being carried on contaminated seeds, soil, or infected plant debris.
The pathogen can overwinter on plant debris in the soil for up to two years, serving as a primary source of inoculum for subsequent infections. Once spores land on a susceptible plant, they germinate and penetrate the host tissue, often through natural openings like stomata or through wounds. Young plants, particularly seedlings, and plants during the fruiting stage are often more susceptible to infection.
Environmental conditions play a role in the development and severity of C. cassiicola diseases. Prolonged periods of high humidity and leaf wetness are favorable for spore germination and disease progression. Temperatures between 20°C and 30°C are generally optimal for the fungus’s sporulation and disease development.
For instance, in greenhouses, C. cassiicola produces a large amount of spores under high humidity (above 90% relative humidity) at night. These spores are then released into the air under lower humidity (below 60% relative humidity) during the day, facilitating widespread dispersal. Understanding these environmental requirements is helpful for predicting disease outbreaks and implementing timely control measures.
Management and Ongoing Research
Managing Corynespora cassiicola presents ongoing challenges due to the pathogen’s adaptability and the development of resistance to chemical treatments. Fungicide applications are a primary method for disease control in many crops, including tomatoes, cucumbers, and cotton. However, the fungus has shown increasing insensitivity to various fungicides, particularly quinone outside inhibitors (QoIs) like azoxystrobin and pyraclostrobin, as well as benzimidazoles and succinate dehydrogenase inhibitors (SDHIs). This resistance is often linked to genetic mutations, such as the G143A substitution in the cytochrome b gene, which confers resistance to QoI fungicides in many isolates.
Ongoing research aims to develop more effective and sustainable control measures. Efforts include studying the pathogen’s genetic diversity to understand its evolution and host specialization. Researchers have identified different phylogenetic lineages within C. cassiicola, with some isolates showing host specificity, meaning they are more aggressive on their original host plant. This understanding helps in developing targeted control strategies.
Identifying new pathogenic factors beyond the known phytotoxin cassiicolin is another area of research. Cassiicolin is a small secreted protein that contributes to the fungus’s virulence, especially in rubber trees, but some virulent isolates do not produce this toxin, suggesting other effectors are involved. Genome sequencing of C. cassiicola isolates is helping to identify these additional putative effectors and understand the genetic basis of disease emergence.
Furthermore, breeding resistant crop varieties is a promising long-term strategy, although research in this area is limited. Some reports indicate resistance in certain tomato, rubber tree, soybean, and cucumber varieties, but developing durable resistance is complex due to the pathogen’s necrotrophic behavior.