Gordon Johnson, Extension Ag Agent, Kent Co.; gcjohn@udel.edu
Pole lima beans are a very profitable crop for market gardeners and produce growers across Delaware. However, a number of problems can come up that can limit yield potentials. Some information follows.
Heat is the major factor in reducing yields in pole lima beans. Hot days and especially hot nights will cause flower and small pod abortion and reduced summer pod sets. It will also cause reduced seed numbers in pods or pods with underdeveloped seeds (flat pods). This is especially a factor in large seeded pole lima bean types (such as Dr. Martins). One strategy to partially avoid this problem is to plant as early as practical in May using plastic mulches to warm soils and providing protection to plants against cold snaps using covers. Early and quick starts will allow for plants to mature and set pods quickly and get some July and early August production before heat really starts to reduce sets. Irrigation in hot weather is critical so that plants are not under any water stress. Misting plants with low volume sprinklers during the middle of the day in hot weather may help but also sets up conditions for downy mildew.
Stink bugs and tarnished plant bugs are a major problem in many areas of the state and can reduce lima bean yields significantly. We have seen an increase of stink bugs in recent years. Stink bugs and tarnished plant bugs pierce young pods with their needle-like stylets, sucking sap out of the pods/young seeds. This causes misshapen seeds or, if severe enough, pod drop. Insecticide sprays are used for control, including a number of pyrethroids. Be careful with some of the pyrethroids because they can cause mite populations to explode by reducing natural controls (predators). See the Commercial Vegetable Recommendation book for recommended products (web link http://ag.udel.edu/extension/vegprogram/pdf/DEvegrecs2008.pdf). Options are limited for non-commercial producers.
Two-spotted spider mites will build up in hot dry weather and can cause leaf bronzing and leaf drop if severe by feeding in leaves with piercing/sucking mouthparts. To detect mites early scout for mite damage on the oldest leaves. The key to reducing mite injury is proper irrigation to keep plants healthy. Mite control materials are available but are limited. See the Commercial Vegetable Recommendation Book for recommended products. Mite control options for non-commercial producers are very limited.
Two spotted spider mites on the underside of a pole lima leaf
Two spotted spider mite adults and eggs
In late July through August, pole lima beans often start to yellow because they are running out of nitrogen. Even though lima beans are legumes, we grow them using nitrogen fertilizers because they tend not to produce enough nitrogen with the nitrogen fixing bacteria. An August side-dressing of nitrogen fertilizer will often be necessary to keep these vigorous vining plants growing and setting pods.
Pod feeding insects such as European corn borer and corn earworm can be problems and may have to be controlled with insecticides. Other insect pests such as thrips, Mexican bean beetle, and leafhoppers can be problems at times.
The major summer disease problems in pole lima beans are downy mildew when the weather is wet, as well as occasional outbreaks of anthracnose and Phomopsis pod blight. Keep informed by reading the Weekly Crop Update on the potential for downy mildew and start controls if advised that weather conditions are favorable.
Air pollution can cause bronzing of lima beans in hot, stagnant conditions with air inversions or in areas with a lot of vehicle traffic.