Black spots on walnuts are a common concern for growers, impacting the nuts’ appearance and quality. Identifying the specific cause is crucial for effective management. This guide details the primary culprits behind these spots, their characteristics, effects, and appropriate interventions.
Common Causes of Black Spots on Walnuts
Walnut blight, a bacterial disease, often manifests as small, dark, angular spots on leaves, catkins, and nuts. These spots can develop into sunken lesions on the husks, sometimes penetrating the shell and the edible kernel. The bacterium, Xanthomonas arboricola pv. juglandis, overwinters in infected buds and catkins, spreading during early spring growth, particularly with frequent, prolonged rain before and during bloom.
Anthracnose, a fungal disease caused by Gnomonia leptostyla (also known as Marssonina juglandis), typically presents as larger, irregular, sunken black spots, often with fuzzy centers on leaves and husks. Initial symptoms on leaves may include yellow-red spots with a red-brown halo, which can lead to premature leaf drop. This fungus thrives in wet conditions and temperatures between 10°C and 32°C, spreading from overwintering spores in infected shoots and fallen leaves.
The walnut husk fly, Rhagoletis completa, causes dark, mushy spots on the husk, often accompanied by larvae inside. Adult flies, about the size of a housefly with distinctive wing bands, emerge from the soil typically around July 1 and lay eggs beneath the husk’s skin. After hatching, the larvae feed inside the husk for three to five weeks, leading to decay and staining of the nutshell.
Impact on Walnut Trees and Nut Quality
Black spots on walnuts can affect both tree health and harvested nuts. Severe blight and anthracnose can cause premature defoliation, weakening the tree and reducing its growth.
Walnut blight can cause young nuts to drop prematurely; if infection occurs before shells harden, kernels may darken, shrivel, or become inedible. Anthracnose can also lead to premature nut drop and result in poorly filled or darkened kernels.
Walnuts affected by the husk fly typically develop stained shells that cannot be sold as in-shell products. While larvae primarily damage the husk, heavy infestations can occasionally affect the kernel, leading to shriveled or moldy kernels. Nuts with black spots from blight or anthracnose are generally safe to consume if the kernel itself is not moldy or compromised.
Prevention and Management Strategies
Managing black spots on walnuts involves cultural practices and targeted treatments. Maintain proper tree spacing for air circulation and promptly remove fallen leaves and nuts, which can harbor pathogens or pests. Avoiding overhead irrigation also reduces disease spread, as wet conditions favor their development.
For walnut blight, applying copper-based sprays is a common approach, with treatments timed to coincide with early shoot emergence. Some walnut varieties exhibit greater resistance to blight, such as ‘Howe’, which can be considered for new plantings, while ‘Ivanhoe’ and ‘Franquette’ are known to be susceptible. Pruning out infected branches can also help limit disease spread.
Anthracnose management involves sanitation, such as raking and destroying fallen leaves to reduce inoculum. Fungicide applications can be effective, particularly in early spring when fungal spores are released.
Controlling the walnut husk fly typically involves monitoring adult fly emergence using traps, allowing for timely application of insecticides before eggs are laid. Timely harvest can also reduce the period during which nuts are susceptible to infestation. Removing fallen infested nuts from the ground prevents larvae from completing their life cycle in the soil, reducing future populations.