Diagnosing Raspberry Problems
Little or no fruit production on established plants can be due to several reasons. One that everyone thinks of is improper pruning – removing canes annually on summer-bearing (floricane) types that only produce on 2-year old canes. Note that fall-bearing (primocane) types should be annually pruned after harvest in the fall.
Perhaps a more common reason for lack of production is viruses. A raspberry planting is productive for about ten years and yields best in the first five years after planting. Various viruses accumulate in plants over time as they are introduced by aphids. These viruses produce no obvious mosaic or visible plant symptoms. Plantings should be removed after ten years or as they yield poorly and a new planting made preferably in new soil. Obtain virus-free planting stock from reputable nurseries. Don’t plant donated rooted suckers from another gardener that are likely virus-riddled. Poor-quality fruit can also be due to viruses such as Crumble-berry virus which causes exactly what its name implies.
Short plants that fail to thrive and produce poorly can be from several causes. Raspberries perform poorly in heavy soils with poor drainage. Soils should be amended before planting and plants probably produce best when grown on raised soil beds. Raspberries are not xeric and require an inch of water per week to be productive. Inadequate or irregularly applied water produces short plants. Crowded canes that self-compete, and poor fertility are further reasons plants may not thrive. Canes should be thinned to 8 inches apart in spring as early growth starts and a general nitrogen fertilizer applied. Inadequate light with less than 6 hours of direct sun contributes to poor fruit production. If part of the day is shaded it’s best in late afternoon.
Yellow spots that turn into bronzed foliage is a sign of spider mite infestation and is more of a problem in hot locations and during hot summers. Provide adequate water and spray leaf undersides to control as needed. Wilted and drooping tops of canes is a sign of raspberry cane borer. Prune off tops at the earliest sign of the insect to prevent the white larvae from tunneling down further into the cane. Destroy pruning containing the insect. Insecticides such as carbaryl (Sevin®) are registered for this pest.
Sunscald and heat are the cause for white druplet disorder on fruit. A few of the druplets or perhaps an entire, sun-exposed side of a berry turns white. The rest of the fruit colors normally. This does not harm the edibility of the fruit, just the appearance. It is an environmental disorder and not one caused by a fungus or bacteria so no sprays are needed. Good leaf cover will be helpful so put efforts into adequate watering and fertilizing.
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Updated Monday, February 10, 2014