I was recently contacted by a Pest Control Advisor (PCA) regarding a lima bean field in the Tracy area. The plants were showing some unusual symptoms, and while the field was nearing harvest, the grower was hoping to get another couple weeks out of it for pod fill. The PCA had already collected plant samples and sent them to a disease lab; the lab diagnosed the plants as having Sclerotinia sclerotiorum, or white mold. Inside the roots of those plants were hard, black sclerotia (Figure 1), which are compact masses of mycelium and characteristic of the disease.

I later visited the field with the PCA in order to assess the situation for myself. The disease appeared to be most apparent in one corner of the field where the water tends to sit a little longer. (This is a furrow irrigated field that has had beans for several consecutive years.) Moist or humid conditions favor white mold, as do heavy canopies. The leaves were showing some unusual brown lesions, and some plants had white mycelium growing on the pods (Figure 2). The most detrimental symptom that we observed, however, was complete infection of the stem at the soil line, causing the tissue to dry up (Figure 3). In this case, the upper canopy looked ok, but I am uncertain, perhaps doubtful that the pods will fill on these plants.

What we can hope is that the disease won't appear on more plants in the next two weeks because there are really no management strategies at this stage in the game. The grower needs to think about how to manage this disease in the future, and rotating out of beans to a non-host crop (like small grains or corn) is probably a good strategy.

Weeds around the field can still harbor the disease, and the hard, black sclerotia can survive in the field for several years. If the grower plants beans again, then it is important not to over-apply nitrogen, which can result in an excessive canopy, limiting air movement.

If wider row spacing is an option, this can also improve air movement in the canopy, which makes conditions less favorable for white mold. Monitoring for the disease should begin at flowering, which is also the best time to apply fungicides (like thiophanate methyl or boscalid) because the plants are small enough for the treatment to penetrate into the canopy.

A grower recently called to ask about the minimum soil temperature for planting blackeye and large lima beans. The University of California, Division of Agriculture and Natural Resources has produced production manuals for both blackeyes and limas. A general guideline for blackeyes is to plant as soon as the weather warms and the soil temperature in the seed zone (about 2.5 inches deep) reaches at least 66°F for the three days following planting. Under these conditions, blackeyes will emerge about three to five days after planting. It is important to try to plant blackeyes early enough that they can set pods before the hottest temperatures of the summer. Planting when soil conditions are still cool, however, could result in a longer emergence time, around 10 days. This longer time to emergence means greater susceptibility to diseases, like Pythium or Rhizoctonia (Figure 1). Blackeyes can be grown on a range of soil types, both coarse and fine textured soils, provided that irrigation is adequately managed to keep the plants from being water-stressed or waterlogged. Blackeyes are moderately tolerant of salinity and can generally tolerate an average root zone salinity of 4.9 dS/m before yield declines are expected.

Similarly, large limas should also be planted when soil conditions have warmed, and a general guideline is when soils have reached 70°F in the seed zone (about 3 inches deep). At these warmer soil temperatures, wireworms will recede deeper in the soil and present less of a problem, and emergence will be quicker and more uniform. The effect of soil temperature on germination and days to emergence is shown in Figure 2. These data show that percent germination improved dramatically when soil temperature reached 68°F, but the average number of days to emergence was greatly shortened when the soil temperature reached 77°F. As with blackeyes, planting limas into warmer soil will help reduce fungal infection. Limas grow best in loamy soils. If grown on finer textured soils, irrigation must be properly managed to prevent waterlogging. Limas have low tolerance for saline soil conditions, and yield declines are expected when the average soil salinity reaches 1.5 dS/m.

In addition to the production manuals previously mentioned, I also consulted UC production manuals produced in the 1950's, including Dry Edible Bean Production in California (1954), Blackeyes: Costs of Production, Suggestions on Growing (1956), and Production of Dry Edible Lima Beans in California (~1951). 

9:00 – 9:30 AM
Five Points Field Station
Lassen and Oakland Avenues
17353 W. Oakland Avenue
Five Points, CA 93624

If you missed the field day,
you can see a video summary here.

Further information is available from Jeff Mitchell

at (559) 303-9689 or jpmitchell@ucdavis.edu.

A pest control advisor recently contacted us to ask what pests he should be scouting for in garbanzo beans. His clients had not grown garbanzos in the past but have some acreage this year. Given recent wet weather and relatively mild temperatures, there are three diseases for which we suggest keeping an eye out.

Ascochyta blight (Ascochyta rabiei, Didymella rabiei) is a particular problem in garbanzo beans in wet years, like what we've been having this year. Ascochyta blight can occur at any stage of growth and on any aerial part of the plant. Brown lesions on the stems can cause damping-off symptoms in seedlings or can cause stems to break. At the advanced stage of the disease, concentric circles of spores will form within brown leaf lesions, and these are a good diagnostic characteristic (Figure 1). These concentric circles can also be seen on seed pods (Figure 2), which can result in poor seed set, seed discoloration, and shrinkage. If these beans are used for seed, subsequent crops can get infected. Management of Ascochyta is through the use of tolerant varieties, crop rotations, certified disease-free seed, always using a seed treatment (Mertect), and foliar fungicides. Foliar fungicides, such as Headline or Quadris, should be applied at the first sign of the disease and reapplied if rainy weather is forecasted.

Alfalfa mosaic virus is another disease that could infect garbanzo beans. Alfalfa mosaic virus has a wide host range and is transmitted by aphids. Different strains of the virus may cause symptoms as varied as necrotic spots on leaves to yellow dots or mottling on the entire plant, which can stunt plants or result in pod distortion. Treatment is not recommended for either alfalfa mosaic virus or for the aphids that vector it. Aphids may taste the garbanzo plants but are either killed or deterred by the acidic exudate of the plant, so it is not economical to treat for them. Instead, planting date is important to avoid aphid flights. There are no patterns of infection in fields. Infected aphids land on plants, transmit the disease and then die, so healthy and dying plants may be next to each other (Figure 3).

Lastly, white mold (Sclerotinia sclerotiorum and S. trifoliorum) may be a problem in garbanzos this year, like Ascochyta blight, because of the wet conditions. White mold may appear as a watery rot on stems, leaves, and pods. White mycelium may grow on the stem near the soil line where conditions are moist (Figure 4). Yellow flagging of leaf tissue may appear where stems have been killed. The fungus spreads under moderate temperatures and especially where canopies are dense. Management is through rotations with non-host crops (small grains, corn), and by applying fungicides, such as Endura, during the flowering stage.

Information on products and practices is for educational purposes only and does not constitute an endorsement or recommendation by the University of California.