Monilinia is a genus of fungi primarily affecting fruit crops, including stone fruits such as peaches, cherries, apricots, and plums, as well as pome fruits like apples and pears. The diseases caused by Monilinia species, often referred to as “brown rot,” can lead to significant crop losses.
The Lifecycle and Spread of Monilinia
Monilinia fungi survive winter primarily within infected plant debris, such as mummified fruits left on the tree or fallen to the ground, and in cankers on twigs and branches. As temperatures rise in spring, these overwintering structures become sources of primary inoculum, producing conidia and ascospores.
Conidia are asexual spores that form on mummified fruits or cankers remaining on the tree, while ascospores develop from mushroom-like structures called apothecia that emerge from mummified fruits on the orchard floor. These spores are readily dispersed by wind, rain, and can also be carried by insects. Infection occurs during periods of high humidity and mild temperatures, with optimal conditions for conidia germination between 15°C and 30°C. A wetting period is necessary for infection to establish, with longer wetness periods increasing the infection rate.
The fungi enter plant tissue through natural openings or, more commonly, through wounds on the fruit surface caused by growth cracks, hail, bird or insect damage, or mechanical injury during harvest. Once established, the fungus rapidly colonizes tissue. Subsequent generations of conidia can be produced within about seven days after initial infection, spreading rapidly throughout the growing season.
Recognizing Monilinia Damage
Monilinia infection manifests in several ways on different parts of the plant. Initial signs appear as blossom blight in spring, where infected flowers wilt, shrivel, and turn brown, resembling scorched tissue. These blighted blossoms may become covered with grayish mold and often remain attached to the twigs, serving as a continuous source of spores for later infections.
As the disease progresses, it extends from infected blossoms into twigs, causing twig dieback. Brownish, oval-shaped cankers form on these twigs, sometimes girdling them, leading to wilting and death of shoots beyond the canker. Sticky, yellowish-brown gum also exudes from the base of blighted flowers or infected twigs, a sign of the disease.
The most significant symptom is fruit rot. This begins as small, circular, light brown, water-soaked spots on ripening fruit, which rapidly expand and darken. As the rot progresses, the fruit softens, and fuzzy, grayish-brown tufts of spores appear on the surface of the decayed areas. Infected fruits eventually shrivel, turn black, and become dry, hardened “mummies” that can either fall to the ground or remain attached to the tree through winter, perpetuating the disease cycle. These damages collectively contribute to substantial crop losses, with potential harvest losses reaching over 30% and up to 90% in severe situations.
Preventing and Managing Monilinia
Effective management of Monilinia infections involves a combination of cultural practices and chemical control measures to reduce inoculum and prevent disease spread. Sanitation is key, starting with the removal and destruction of infected plant parts. This includes pruning out dead wood, blighted twigs, and cankers, and ensuring all mummified fruits, whether on the tree or on the orchard floor, are collected and removed from the site. This practice significantly reduces the amount of fungal inoculum available for the next growing season.
Proper pruning improves air circulation within the tree canopy, reducing humidity and creating less favorable conditions for fungal growth. Avoiding overhead irrigation minimizes the spread of spores, as water splash is a primary means of dispersal for Monilinia conidia. Preventing physical damage to fruits during harvesting is important, as wounds serve as easy entry points for the fungus. Some fruit varieties exhibit greater resistance or tolerance to Monilinia; selecting these for new orchards is a proactive measure.
Chemical control involves the strategic application of fungicides. Applications begin during bloom to protect blossoms, with further applications needed through harvest to protect fruit. Fungicides can be protectant, forming a barrier, or systemic, absorbed into tissue. The timing of these applications is important, correlated with specific growth stages and prevailing weather conditions that favor disease development. An integrated approach combining these cultural practices with targeted fungicide applications is the most effective strategy for managing Monilinia and minimizing its impact on fruit crops.