Crumbly fruit are caneberry fruit that do not stay together very easily and crumble or fall apart when picked or handled. Crumbly fruit is usually a result of incomplete drupelet set or drupelets on the fruit that have not filled (drupelets are the individual fruit of which in aggregate compose a raspberry or blackberry fruit). Unfilled and missing drupelets leave spaces in the fruit, causing a collapse of the surrounding drupelets into this space.
There are several causes of crumbly fruit in caneberries, which include:
- Poor pollination: Extreme weather conditions, either hot or cold, affect pollination and drupelet set. Honeybees do not move very well, if at all, in extremities of weather, resulting in uneven pollination that causes crumbly fruit. The current dearth of wild honeybees does not bode well for growers not employing professionally managed bee hives.
- Vascular injury: Vascular injury from severe cold or physical damage can impede water uptake in the caneberry plant, causing wilting, plant stunting, as well as crumbly fruit. Drupelets of the fruit need water to be filled; interference with water transport into the fruit results in uneven drupelet fill.
- Viruses: Viruses that can cause crumbly fruit in certain varieties include: Raspberry leaf curl virus, Tomato ringspot virus, and Raspberry bushy dwarf virus. Plants can be infested with viruses at the propagation stage, so it is imperative that growers only work with and purchase plant stock that is certified virus-free.
- Dryberry mite: Although not a problem on the Central Coast of California, dryberry mite is another cause of crumbly fruit in caneberries. Normally infesting the leaves of the developing primocane, this eriophyid mite can move to fruit during periods of high populations. Damage to fruit is distinct from crumbly fruit symptoms resulting from other causes because fruit infestation causes some drupelets to ripen early, leaving a badly misshapen fruit that does not necessarily crumble.
The pattern of crumbly fruit development in individual caneberry plants as well as in the field is very useful in indicating the probable cause. Viruses infest the whole plant, and infested plants have crumbly fruit up and down the cane. Extremities in weather most often occur over a few days, so only those flowers and subsequent fruit exposed to these conditions will express crumbliness. Crumbly fruit then would be found through the whole field only at a certain height of cane or length of fruiting lateral in the case of exposure to extremes in weather. In the case of improperly done plant propagation, problems can be transmitted from an initially low number of plants to large numbers of nursery stock. In the field this will manifest itself as entire blocks that express the problem evenly throughout.