Dimerella

Dimerella is a genus of lichenized fungi that are frequently observed growing on the bark of fruit trees and ornamental shrubs. While they are not obligate parasites, their presence is a clear indicator that the host plant is suffering from underlying issues such as poor aeration, lack of vigor, or neglected canopy management. They survive by colonizing the bark surface rather than feeding on the sap.

The biological nature of this organism involves a symbiotic relationship between a fungus and algae. It targets various woody species, particularly apple, pear, and plum trees. While it does not directly consume plant tissues, the thick crusts formed by these lichens significantly impede the tree's ability to undergo essential gas exchange through the bark pores (lenticels), putting the tree under physiological stress.

Symptoms appear as small, often bright yellow, orange, or green apothecia (fruiting bodies) scattered across the tree trunk and main branches. As the colony matures, it forms a crusty layer that can spread rapidly over the bark. This layer creates a rough, uneven texture that traps moisture and dirt, potentially leading to further bark decay.

Environmental factors play a crucial role in the development and spread of Dimerella. High humidity, low air circulation, and deep shade are the primary drivers for these lichens. Orchards that are not pruned correctly are especially prone to colonization, as the trapped microclimate within the dense canopy provides an ideal environment for lichen growth and reproduction.

The economic impact of lichen infestation is indirect but significant: it provides a refuge for pests such as scale insects, mites, and even overwintering spores of fungal pathogens like apple scab. Protection strategies focus on maintaining tree vigor through proper fertilization, regular pruning to ensure sunlight penetration, and physical cleaning of the bark, often followed by an application of copper-based fungicides.