Stamen blight of caneberries—Hapalosphaeria deformans
Stamen blight, also called anther and stigma blight, is an uncommon fungal disease of caneberries. When flowers become infected, this causes fruit to mature unevenly and become at least partly dry and seedy.
Identification
Stamens are the male fertilizing organs in a flower that produce pollen. The stigma is the female part of a flower that receives the pollen during fertilization. With healthy caneberry flowers in full bloom, the pale green to yellowish stigma is at the center of a blossom. It is surrounded by a ring of white-stalked stamens each with a brown anther at the tip.
When pathogen-infected and decaying (blighted), both stigmas and stamens become covered with the powdery gray to white growth of fungal mycelia. Tiny black fruiting bodies (pycnidia) develop on the infected anthers and stigma. Diseased flowers may appear larger than healthy flowers because infected stamens become flattened against the petals and sepals. Infected anthers become discolored and swollen. Petals of infected flowers may turn brown before they would naturally senesce (age and drop).
Life cycle
The fungal cause of stamen blight spreads as spores. The spores are moved by wind, splashing rain or overhead irrigation, mechanical (physical) contact with infected parts, and bees and other insects that visit the blossoms.
Infection by H. deformans prevents the production of pollen. But infected flowers may produce fruit if cross pollination occurs from healthy blossoms. However, such fruit are generally misshapen with only a portion of the drupelets being normal and other drupelets being dry and seedy.
Damage
If caneberry flowers have been infected with H. deformans, the berries fail to properly mature and become unpalatable. Infected fruit are more firmly attached to the receptacle than normal. This makes harvesting more difficult and results in crushed, wet fruit during picking.
Solutions
Purchase caneberries from a reputable commercial suppler that provides disease-free plants. Plant caneberries on a berm or mound of raised soil. Do not crowd the plants and prune them to thin canes. Control nearby weeds. These practices improve air circulation and reduce humidity around plants, which helps to control various caneberry diseases.
Using an alternate-year fruiting system where canes are trained up (trellised) as they grow can also help to reduce this disease. Use drip irrigation instead of sprinklers and water early in the day so drying occurs more quickly. Growing caneberries inside a macrotunnel (hoop house covered with clear plastic) also helps to control many caneberry diseases because the surface of plants is kept more dry.
Provide caneberries with a good growing environment and appropriate cultural care so they grow well and are more tolerant of disease damage. Remove any wild caneberries near those grown for their fruit because wild caneberries can be a source of various caneberry pathogens. Note that where host plants occur in riparian areas there are regulations that limit vegetation removal. Before managing vegetation along waterways, contact the California Department of Fish and Wildlife to learn of any rules that may apply to your situation.
Whether applying fungicides available to home gardeners will help to control stamen blight appears to be unknown. Potentially effective products include Bordeaux mixture, other copper fungicides, and sulfur. Make any application in spring during the delayed dormant season, when buds swell but before they open. If the disease becomes common spray again in August.
Adapted partly from 1982 Spray Guide for Cane Fruits in Oregon, Oregon State University (PDF) and Diseases of Raspberries and Erect and Trailing Blackberries, U.S. Department of Agriculture. |
