Do your bean leaves look like they’ve sprouted freckles or gotten the measles? It’s probably bean rust caused by the fungus Uromyces appendiculatus. Spots appear raised on both the upper and lower leaf surfaces and may have yellow borders. As disease progresses, leaves may brown and fall from the plant. Of course, this can impact crop production. Bean pods may also become infected, further affecting harvest quality.
The rust fungi have interesting life cycles that can involve two unrelated hosts and up to five different spore types. In the case of bean rust, there is only one host, making this an autoecious rust, but all five spore types occur. The fungus overwinters in the thick-walled teliospore stage associated with infected plant debris. In the photo below, the darker spores are teliospores (first spore type).
In the spring, coinciding with favorable weather and the time when young bean plants would be susceptible, the teliospores germinate and produce basidiospores (spore type 2) that initiate infections on bean leaves (and sometimes stems). As this infection progresses within the leaf, tiny structures called pycnia containing pycniospores (spore type 3) are produced on the leaf surface. Pycniospores are extruded in a sweet liquid attractive to some insects. When the insects feed on the liquid, they pick some up on their external body parts and move the pycniospores from one pycnium to another. When pycniospores of one mating type are introduced into a pycnium of the opposite mating type, it leads to the production of the fourth spore type, the aeciospore, on the opposite (lower) side of the leaf. The aeciospores in turn infect the leaf and this infection leads to the production of new pustules containing urediniospores (fifth spore type), the orange or rust colored spores that give this disease and other rust diseases their common name. Urediniospores are produced throughout the growing season and, under favorable conditions (moist), can cause additional infections. Late in the season, the thick-walled teliospores are produced in the same pustules as the urediniospores as cooler weather approaches, completing the fungus’ life cycle.
In my garden, rust occurred on the pole beans but not on the bush beans. This is not surprising because many bean cultivars have resistance to at least some of the many strains of the bean rust fungus. Control measures include thorough removal of infected plant debris (don’t compost), resistant varieties, and minimizing leaf wetness by providing for ample plant spacing and avoiding overhead irrigation. Spores require moisture to germinate and cause new infections. To minimize spread, avoid working among the plants when they are wet. Fungicides labeled for bean rust are available and, if needed, should be applied beginning when symptoms first appear and at the interval recommended on the label. When using pesticides, always read and follow the label carefully.
As a quick side note, I found two-spotted spider mites on my beans, too. Not only that, but the larvae of a beneficial predator of the spider mites, the midge Feltiella acarisuga was noted as shown in the below photos.
Symptoms of spider mite infestation include a ‘stippling’ effect on the leaves from many tiny spots at the mites’ feeding sites, and especially on the lower leaf surfaces, and signs include webbing, mites and eggs. These mites are favored by hot, dry conditions so this summer was great for them. They are often kept in check by naturally occurring parasites and predators that can be eliminated by the use of insecticides/miticides resulting in a more severe mite outbreak following application.
J. Allen, Assistant Extension Educator