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Lack of Roots Cause Raspberries to Helplessly Wither

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Washington State University plant pathologists have identified over thirty different root rot fungi that can infect raspberries in our coastal area. Plants infected with rot fungi not only have rotted roots, but also lack fibrous roots. After hot, dry periods older leaves may wither or become bronzed or scorched. Attached leaves droop. Fruit stems usually are shortened and berries, if formed, remain small and often wither before ripening. Once the root system begins deteriorating, new roots may arise from above decayed ones in summer.
Although this makes plants look as though they have recovered, the new roots are often weak and lack lateral development. The new roots in turn will become infected during the cold, wet weather this fall and winter so that the plant progressively declines and is unproductive. No Pacific Northwest raspberry varieties are very resistant to the problem; however, Chilliwack, Meeker, Sumner and Summit are moderately resistant. Young Meeker plants are very susceptible, while mature plants seem to have some tolerance.
Home gardeners can reduce root rot problems by using certified planting stock and setting plants in fertile, well-drained soil, which has a 3 to 4 foot deep water table in winter. Select a site, which has not grown raspberries or other bramble fruit for several years. Place plants in beds that are raised so that the top of the bed is at least 12 inches above the surrounding soil. Slope soil away from the center of the planning bed to the alleyway between rows.
The fungicides Aliette, Agri-Fos, Fosphite, Phostrol and Ridomil Gold SL are all registered for use to prevent raspberry root rot. Most require multiple applications and must be used well in advance of harvest.

What Causes my Raspberries to Crumble?

Anyone who’s ever grown raspberries knows that fruit with only a few druplets tends to crumble when it’s picked. This is the result of abnormal fruit development. Normal flowers on a raspberry plant have between 100 to 125 pistils. Each is able to produce a seed and a druplet. In normal berries, from 75 to 85 druplets usually develop. If appreciably fewer than 75 druplets develop, the berry does not hold together and crumbles as it’s pulled from the plant.
Crumbly berries have a number of possible causes:

  • Lack of nutrients:  Anything that seriously interferes with plant nutrition –such as drought, extremely low fertility, or damage to roots or crowns from nematodes, symphylans, root rots, crown gall, crown borer, flooding in winter, or cultivating too deep may bring about crumble. If plants with crumbly berries also have cane buds that fail to grow or short, stiff laterals with odd-shaped leaves, boron may be lacking. Taking a soil test will validate if Boron is needed and how much is needed to correct the deficiency. Some varieties tend to crumble more than others. The Tahoma and Latham varieties often bear crumbly fruit. Some seedling and some clones of the Sumner variety produce crumbly fruit. Occasional plants of most varieties apparently mutate to a crumbly condition. 
  • Diseases: Viruses may cause failure of flowers to function or seeds to develop abnormally even though growth appears to be normal. The raspberry mosaic complex has been associated with crumble, but specific varieties associated with raspberry crumble have not been identified. In certain instances, bacterial and fungus diseases have been suspected of contributing to crumble. 
  • Other Causes:

o    Lack of bee activity and lack of pollination may result in crumbly berries.

o    Chemical damage to flowers from in-bloom applications of insecticides or fungicides could damage anthers, pistils, or pollen.

Our raspberries have finished producing fruit for the year.  When and how should we prune them back?

Caneberries, like raspberries, loganberries, blackberries, and blackcaps need care after they are harvested. Three things need to be done to keep your plants healthy and ready for next year’s crop.

The first thing to do is prune.  Remove all the canes, which bore fruit this season. This pruning exposes the new canes (those that will bear fruit next year) to the light and gives them a chance to develop.

Retain 10 to 12 of the healthiest canes for the next year’s crop. Remove the top half of the cane from fall-bearing cultivars after fruiting is over, or remove the entire cane at ground level. Leave the lower half of the cane for a summer crop the following June, or remove the cane entirely for only a fall crop each year. Because the fall crop on fall fruiting varieties is superior to the summer crop, WSU horticulturists advise cutting all canes to the ground in mid-October.

The last thing to do is very easy and also very important.  Stop watering the new canes! This will allow growth to slow and the canes to harden up before the first frost. Do not fertilize canes at this time of the year. That will stimulate unwanted growth.

Editor’s Note: This article first appeared on www.hometowndebate.com 7/18/12. If you would like to respond to this story, go to hometowndebate.com
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