Brown rot blossom and twig blight—Monilinia spp.
Brown rot is a common and serious blossom and fruit disease of almond, apricot, nectarine, and peach. It occurs less seriously on other stone fruits and quince. The fungi kill flowers, girdle and kill twigs, and cause maturing almond nuts and fruit to decay.
Identification
Brown rot is recognized by its characteristic symptoms as detailed below under Damage. In summary blossoms and the associated shoot wilt and die. Gum commonly exudes at the base of infected flowers. Cankers develop on the blighted twigs and have tan centers with dark margins. During foggy, high-humidity, or rainy weather, gray to tan spore masses form on diseased flower parts and twig cankers. Later in the growing season, fruit and nuts can develop discolored spots of decay. Spore masses may then develop on the infected fruit or nuts.
Lookalikes. Bacterial blast, blight, and canker caused by the Pseudomonas syringae bacterium produces similar blossom death, cankers, gumming, and shoot dieback on almonds and stone fruit. Unlike the clumps or masses of gray and tan spores that develop on tissue diseased by brown rot when it is humid or wet, bacterial blast produces no visible spores. Shoots killed by brown rot turn mostly brown to tan while those killed by bacterial blast generally turn black.
Citrus fruit that are almost mature can also develop a disease called brown rot. This malady is caused by Phytophthora species, oomycete pathogens that also cause Phytophthora gummosis and Phytophthora root rot.
Life cycle
The seasonal cycle of brown rot is that the fungi survive and overwinter in diseased flowers and shoots and old fruit and nuts (mummies). Spore masses (sporodochia) develop on these infected plant parts in late winter and are blown or splashed to infect new blossoms in spring. Spore production and infection are favored by fog, high humidity, or rain, and temperatures in the mid-70s°F during bloom. Little or no blossom blight develops if blossoms do not become wet during bloom.
Damage
The first symptoms of brown rot are the browning and withering of blossoms. Dead blossoms commonly remain attached to twigs for a long time. Sunken, brown areas (cankers) develop on twigs at the base of some of the infected flowers. The cankers cause leaves outward or upward from the canker to wilt and discolor yellow then become brown or tan and dead.
Brown to yellowish sticky droplets of gum may exude from the base of dead flowers and the bark of infected twigs. During wet weather, gray or tan clumps of velvety spores form on diseased blossoms or cankers. Large numbers of flower-bearing twigs are killed when the disease is severe.
Later in the season when Monilinia spores land on injured or moist fruit or nuts, brown or tan circular spots develop and enlarge. Gray or tan masses of spores are then produced on the discolored spots. The infected tissue remains relatively dry and firm. Fruit rot can develop on the tree or after harvest during fruit storage.
Fruit and nuts are most susceptible to infection by the brown rot fungi when they are mature. Most rot develops during the month before harvest, although rot occasionally may develop on green fruit in early summer. Almond hulls may be infected from the time they split until they dry.
Solutions
Where available, choose plant varieties that are resistant to brown rot. Provide plants with a good growing environment and appropriate cultural care to keep them growing vigorously.
Promptly remove and dispose of diseased shoots during dry weather to reduce the development and spread of brown rot spores that cause new infections. Use sharp tools to make each cut just outside the branch collar and branch bark ridge. After working on an infected tree, clean tools and dip or spray them with a 10% bleach solution, 70% isopropyl rubbing alcohol, or a commercial disinfectant as directed on the product label. During dry weather, prune young trees to reduce interior branches and allow for good air circulation in the canopy.
Use drip or furrow irrigation or low-angle sprinklers to avoid wetting blossoms, foliage, and fruit when irrigating. Avoid overfertilizing trees because excess nitrogen makes shoots and flowers more susceptible to infection. Manage other diseases and pests that injure fruit or nuts because such wounds increase the chances of brown rot infection.
When green fruit are present, inspect them at least weekly for symptoms of decay. Remove and dispose of affected fruit before spores form on them. As fruit ripen, continue to examine them and cull those that are rotting. Also dispose of any that have fallen to the ground. If numerous fruit are found affected, harvest fruit early and refrigerate them.
During harvest, handle fruit carefully to avoid injury. Consider discarding all fruit that show decay symptoms to reduce the spread of the pathogen during fruit storage. Cool fruit as quickly as possible after harvest and periodically inspect it during storage to detect potential disease development.
Where brown rot has been a problem during previous seasons, fungicide can be sprayed to thoroughly cover the blossoms during bloom. For example, myclobutanil or thiophanate methyl can be sprayed at 20 to 40% bloom. If high humidity or heavy rainfall occur afterwards, a second application can be made at 80 to 100% bloom.
Adapted from Integrated Pest Management for Stone Fruits and Pests of the Garden and Small Farm: A Grower’s Guide to Using Less Pesticide, University of California Statewide Integrated Pest Management Program (UC IPM). |
