Pole lima beans are a very profitable crop for market gardeners and produce growers across Delaware and also have an enthusiastic following among home gardeners. However, a number of problems can come up that limit yield potential. We have getting calls about three major issues in recent weeks.
Downy mildew, caused by Phytophthora phaseoli, affects pods, shoot tips, racemes (flower stalks) and sometimes petioles, but there are rarely symptoms on leaves. Downy mildew produces a white cottony growth on affected plant parts. This season’s wet weather has favored Downy mildew’s development and spread.
Downy mildew on a pod of Dr. Martin pole lima bean.
Downy mildew on shoot tip, petiole and leaf of Dr. Martin pole lima bean.
Some other lima pathogens can also produce white mycelial growth on pods (white mold, pod rot, Pythium) but these rarely affect trellised pole beans because their spread depends upon proximity to infested soil.
Phosphonate fungicides containing potassium phosphite salts (i.e. Phostrol, Agri-Fos) are effective if applied when downy mildew symptoms are first observed. Some of these products are available in smaller quantities (quart) that are suitable for use by growers with small farms or home gardeners. Ridomil Gold is an option for commercial applicators. As a cultural control, growers may consider removing symptomatic pods from the field in addition to spraying. Plant debris should be removed at the end of the season to prevent overwintering of downy mildew, especially if limas are grown in the same area year after year. If you save your own seed, do not save seed from diseased pods.
Three of our common stink bug species are pests of lima bean: green stink bug, brown stink bug, and brown marmorated stink bug. Stink bugs can reduce lima bean yields significantly. These insects feed by piercing developing pods with their needle-like stylets, sucking sap out of the pods or young seeds. This causes misshapen seeds in more developed pods and dropped pods when seeds inside young pods are killed. Hot weather will also cause poor pod set, but if conditions are cool and pods and flowers are dropping from plants, stinkbugs are frequently the cause of the problem. Scouting for stinkbugs in a mass of thick pole be foliage can be difficult. You may want to instead look for the damage they cause to confirm they are causing a problem.
Stinkbug feeding damage on a lima bean leaf. This type of damage probably does not cause yield loss, but it will help you to know that stink bugs are present.
Stink bug feeding damage on baby lima pods (left) and undamaged pods at a similar developmental stage. Stink bug feeding can kill developing seed and cause fluffy white growth inside of pods at the feeding site.
Pole lima bean pod with stink bug feeding damage indicated by arrows.
Stink bug eggs on a pole lima pod.
Recently hatched stink bug nymphs on a pole lima pod.
Green stink bug nymph
Brown marmorated stink bug nymph
Adult green stink bug.
Adult brown stink bug.
Insecticide sprays are used for control. Commercial applicators may consult the Mid-Atlantic Commercial Vegetable Recommendations for insecticide options (http://extension.udel.edu/ag/vegetable-fruit-resources/commercial-vegetable-production-recommendations/). Home gardeners and small scale growers can look for products containing bifenthrin which are labeled for use on lima beans. Follow the product directions for application and days until harvest. Such products are available in some garden centers. Be careful with some of the pyrethroids because they can cause mite populations to explode by reducing natural controls (predators).
The third issue we have seen is yellowing of leaves and poor vine growth not related to insects or disease. This is often due to nitrogen deficiencies in mid-summer. In addition, in a wet year such as 2017, poor root function and denitrification resulting from saturated soils will lead to poor vine growth, yellowing, and poor pod set. Severe N deficiency in lima beans will be seen as an overall yellowing of plants with lower leaves often dropping off as N is mobilized from the oldest leaves to support the new growth at growing tips. Less severe N deficiency will be seen as a lighter green color than normal with lowest leaves most affected. There are other potential causes for yellowing in lima beans including low pH leading to magnesium deficiencies and excessively high pH leading to micronutrient deficiencies, most commonly manganese.