One of the most challenging and gratifying aspects of extension work is trying to answer questions posed by citizens. The variety is endless. I don’t have all the answers and that is what makes it interesting. Correct diagnosis and useful recommendations are at the heart of extension. The questions are determined by the seasons and several questions have been asked relative to raspberry issues. The following are the most common concerns.
Our soils tend to be high pH (alkaline). This leads to the iron in the soil being tied up and unavailable for plant uptake. Some plants are more susceptible, and raspberries are one. While other problems can cause chlorosis (yellowing), iron deficiency is the most common. In early stages, the veins within the leaves remain green while the tissues between veins progresses from yellow to near white. As the deficiency is prolonged, leaf edges die and become brown. The plants themselves fail to thrive and raspberry harvest becomes a great disappointment.
Reduce iron chlorosis through deep, infrequent watering and proper fertilization. The addition of iron in our soils is likely only effective with EDDHA iron chelate. That is the only iron supplement that maintains plant-availability when soil pH is above 7.5 (which most of our soils are).
Raspberry Horntail Larvae
Rest assured, you DO have raspberry horntail larvae if you grow raspberries. Adult raspberry horntail are a dark-colored wasp. Females insert eggs into new cane growth. Eggs hatch and the larvae feed down through the center of the stem. The infected canes wither and droop at the top. Sometimes the cane rebounds and puts out new growth as the larvae feed lower into the stem. Your best solution is to cut the top 6-12 inches of the cane and discard it with the larvae inside. This reduces the population of horntail wasps. If done early (June/July) the canes with sprout from secondary buds and still produce fruit in the fall or following year, depending on variety.
One of the few tradeoffs of drip irrigation is the increase of spider mites. These tiny arachnids thrive in hot, dry, dusty conditions. They feed on lower leaves first, causing the leaves to ‘bronze’ or ‘burn’ as leaf tissue is damaged. Small populations are irrelevant, and you should only treat if damage is significant. If you suspect spider mites, tap the leaves over a white sheet of paper and inspect the paper for moving specks of ‘dust.’ Treatment is best done with neem oil which is a natural insecticide extracted from the neem tree. You can also use miticides or general insecticides. Miticides are pricey and harder to acquire and general insecticides will collaterally remove beneficial predators and other insects. Leaf hoppers can cause similar damage, but the insects are much more visible and the damage they cause gives more of a speckled appearance to the leaves. Treatment can be the same.
Inappropriate fertilization and/or irrigation will exacerbate any nutrient deficiency or pest damage. Plants easily manage low to moderate pest populations if they are healthy. Do your part to keep your raspberries happy and they will reward you back.