Crumbly raspberry disorder being seen across Michigan

Small, misshapen, crumbly fall red raspberry fruits can have several causes.

Late last week, inquiries from fall red raspberry growers across Michigan were made asking what was happening to their raspberry crop that is beginning to ramp up in the harvest window. Growers reported seeing berries that are much smaller than usual, misshapen and crumbling when harvested. The canes and leaves appear normal in terms of their growth and color.

Normal raspberry flowers have between 100 to 125 pistils. Each pistil is able to produce a seed and a drupelet. In normal berries, 75 to 85 drupelets usually develop. If appreciably fewer than 75 drupelets develop, the berry does not hold together and crumbles as it’s pulled from the plant. Crumbly fruit usually contain fewer drupelets than normal, so they are small. The berries are of such poor quality that they are not marketable for fresh market berries, pick-your-own sales and even for the processing market.

There are many potential causes of crumbly fruit. Many times growers have difficulty sorting them out. The list of causes of crumbly raspberry disorder is long; here are a few of the most common causes suggest by Michigan State University Extension.

Poor pollination

Poor pollination causes crumbly fruit because a full complement of drupelets fails to develop. Raspberries are self-fruitful, but bees are necessary to move pollen from the anthers to the stigma for full fruit set. Inadequate numbers of bees, both native and introduced, can cause small and crumbly fruit. Careless pesticide applications can reduce pollination by killing foraging bees. Some pesticides may also repel bees for some time after applications.

Poor pollination weather can potentially limit bee activity, but raspberries are extremely attractive to bees, so weather has to be unusually poor. Extremes in daily temperatures (too cold or too hot) over several days will contribute to crumbly berries; this could affect the bees or pollen tube growth.  Most often these extremes 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.

Tarnished plant bugs

Tarnished plant bugs cause crumbly fruit by feeding on the flowers or developing fruit. As a result, some drupelets do not develop and berries are irregular in shape, small and crumbly.  To prevent this damage, scout for the pest early in the season and apply appropriate controls. This pest also feeds on a variety of fruit crops. This season very few tarnished plant bugs were found by growers and scouts in any of our fruit crops.

Two-spotted spider mites

Two-spotted spider mite infections have been referenced in the literature occasionally to contribute to crumbly berries, but there is not a clear link here. This year, two-spotted spider mite populations have generally been very low.

Virus diseases

Virus diseases are a potential cause of crumbly berries. There are three known viruses; tomato ringspot virus, raspberry bushy dwarf virus and raspberry leaf curl virus. All three are systemic diseases with no cure other than removing the plants. Tomato ringspot is spread by dagger nematodes so the disease tends to start in certain locations and spread slowly to neighboring plants. Raspberry bushy dwarf virus is spread by bees carrying infected pollen. It can become widespread in plantings in a short time. Raspberry leaf curl virus is one of the most damaging viruses in raspberries and is spread by the small raspberry aphid (Aphis rubicola).

In the case of virus diseases, affected bushes tend to be spotty or clustered in the field and express foliar symptoms such as stunting, leaf crinkling or unusual color patterns, although plants with tomato ringspot virus often appear quite normal aside from having low vigor. In addition, the problem would become gradually worse over the season or over several years. If the plants look healthy and berry crumbliness came on suddenly and fairly uniformly, other causes are more likely.

Botrytis

Botrytis or gray mold is common on ripening raspberry fruit, but this fungus can also infect flowers if bloom occurs during rainy periods or when dew is particularly heavy such as in the fall. Bloom infections usually kill whole flowers and no fruit are set, but partial damage can also result in small crumbly fruit. Based on where we at in harvest this year, this is most likely not the problem.

Boron deficiency

Boron deficiency can also cause poor fruit set and crumbly fruit. This nutrient is particularly important for pollen germination and pollen tube growth. Deficiencies are most likely on very sandy soils. Check boron levels by submitting leaf samples for nutrient analysis. If levels are low, apply 0.5 to 1.0pounds of boron as a soil application in the spring, or 0.5 pounds B as a foliar spray prior to bloom in summer bearing varieties or in mid-summer on fall-bearing types. Be careful not to apply too much boron; excess levels can kill plants. The chances of this shortage being seen just this year and in so many locations across the state make this cause unlikely.

In our opinion, the crumbly berry disorder is most likely caused by a combination of poor pollination and extreme weather conditions (too warm or too cold during bloom).

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