There are many causes of poor formation and misshaping of raspberry fruit, including bad weather, genetics or viruses. However, the most common cause is a lack of pollinators, such as domestic and wild bees, during the flowering period. Although raspberry flowers are self pollinating, bee activity is still responsible for 90-95% of pollination. Generally, two strong hives are recommended per acre of raspberries.
To understand how poor pollination can result in misshapen fruit, it is important to view the nature of the raspberry flower. The flower is composed of 100-125 pistils, to which the pollen must be transferred to create a mature seed and the druplet surrounding the seed.
Around 75-85 druplets compose a raspberry fruit and each individual druplet has the same structure as a plum, cherry or peach. If each and every one of these druplets is not pollinated, the overall integrity of the fruit is compromised. This is because the immature druplet stays small, does not contribute to the structure and strength of the whole, and the resulting fruit is misshapen and crumbly (i.e. falls apart easily).
The pictures below demonstrate what improper pollination will look like in raspberry. In this case, many of the druplets at the tip of the fruit have not apparently been pollinated, and are staying immature and small. Beyond that, one can see clearly in the second picture that there is excess nectar accumulating on the flower, simply because it has not been collected by bees. Later, this nectar is certain to cause problems with sooty mold and other fungi, which flourish in this nutrient rich medium.
There are several things raspberry growers must know of when managing bees:
1. Hives must be strong. There should be many frames of brood in the spring and lots of bees in the boxes. Anything less will be an indication of a weak beehive. Flight activity should be heavy, and the raspberry field should be full of bees, not just a bee here or there.
2. Raspberry flowers are supposed to be quite attractive to honeybees. However, the possibility remains that they are being attracted to other flowers in the vicinity, either because they are more attractive, or because the raspberry crop is overwatered, meaning the nectar might be thin and not attractive. If a grower’s bees are not going to his or her field, they should be followed out to see where they are going.
3. Some pesticides, if not directly toxic to bees, may yet serve to repel them. Growers should take note of any changes in bee behavior or hive strength following pesticide application.
I thank Dr. Eric Mussen from UC Davis for his valuable insight and contributions on this issue.
Mark Bolda and Steven Koike, UC Cooperative Extension
The recent spate of rainy weather in the Watsonville-Salinas strawberry production district has created ideal conditions for fruit rot caused by Phytophthora. Significant fruit loss is being observed in some fields, and indeed several samples were recently brought to the UCCE diagnostic lab in Salinas and tested positive for Phytophthora.
Phytophthora rot of fruit, known as leather rot, can affect immature green or pink fruit (photo 1) as well as fully ripened red fruit (photos 2 and 3). Symptoms consist of off-white, gray, or yellow-brown lesions. Lesions often begin as localized, circular to oval shaped infection areas that later enlarge into irregularly shaped patches that can affect much of the fruit. The infected area is very soft to the touch. Fruiting bodies or other fungal structures are not seen externally on these lesions. Upon examining internal fruit tissues with a microscope, the diagnostic appearance of the mycelium (relatively thick hyphae that lack cell cross walls) can be observed (photo 4). Phytophthora cactorum is the primary species causing this fruit rot; this same pathogen can also cause crown rot disease.
Growers and PCAs should be reminded that anthracnose fruit rot, caused by Colletotrichum acutatum, may appear similar to Phytophthora infections. However, anthracnose fruit lesions are sunken, oval to round, firm in texture, and brown to dark brown in color (photo 5). In advanced stages and under suitably wet conditions, anthracnose lesions may show white mycelium and salmon to orange colored spore masses of the fungus.
Furthermore, as we work with identifying SWD, it is becoming easier to separate female SWD from other vinegar fly species. The dark line delimiting the serrated edge of the ovipositor (egg laying organ) is quite obvious on many specimens under the 20x magnification available with a good hand lens. Nevertheless, it is important that researchers and other insect professionals become familiar with this identifying feature under a dissecting scope before endeavoring to do it with a hand lens.
There have lately been questions about trapping for spotted wing drosophila, Drosophila suzukii, in strawberry.
While there have been few reports of severe infestation of spotted wing drosophila in California fresh market strawberries, nevertheless many growers want to know how to effectively detect this pest. The trapping system previously described for raspberries is designed for a tall plant with plenty of cover and a trellis to hang it out on. Strawberries, by contrast, are short in stature and do not provide much in the way of shade, in addition to having no trellis to support a trap.
The photos below are of a system of trapping for spotted wing drosophila used successfully in British Columbia. The trap holding the same yeast-sugar-water medium as previously described for raspberries is a flattish 750 ml container with holes approximately of ½ inch in diameter in the lid. Since the trap is on the ground, smaller size holes in the lid may be more effective in excluding carabid and other ground beetles. It is partially dug into the soil to keep it from overturning as well as to maintain the bait at a lower temperature. Note also the placement of the container well into the shade of the strawberry plant.
Thank you to Entomologist Tracy Hueppelsheuser from British Columbia Ministry of Agriculture and Lands (BCMAL) for sharing this information.