Ascochyta blight (Ascochyta rabiei, Didymella rabiei) was recently found in volunteer garbanzos in the Sacramento Valley, so keep an eye out for this disease in your production fields and be sure to control all volunteer garbs on your farm (yes, these volunteers are now long gone).

Symptoms

Ascochyta is a serious fungal disease of garbanzos that can kill seedlings and significantly reduce yields and seed quality in older infected plants. There are no other known crop or weed hosts. This disease is favored by cool, rainy weather and only infects the above ground parts of the plants. Early symptoms include circular light brown lesions on the plants that cause stems to weaken and break. Over time small, black, raised spots (pycnidia-like spores) will form concentric rings in the lesions. Developing seeds can become infected and, if used for seed, can lead to early infections in the next crop. The fungus survives outside or inside the seed. 

Comments

The fungus survives only on crop debris (and seed), not in soil. It does not produce resting spores to survive in soil. How long the debris can persist before decomposing depends on the climatic conditions. It lasts longer in drier conditions. During wet weather, spores are released and infect plants. Moderate temperatures (68º to 77ºF) and wet weather are optimal conditions for severe disease development. 

Management

• Plant Ascochyta tolerant varieties.
• Keep stands healthy, as disease resistance can break down in stressed fields.
• Always use certified, disease free seed.
• Thoroughly incorporate infested garbanzo residue to hasten decomposition and to minimize the possibility of spore production and dissemination.
• Rotate to other crops for two or three years. That will eliminate inoculum in the soil because the fungus will not survive in the absence of a garbanzo host.
• Avoid early plantings (November and early December) since they result in large plants and a thick canopy, which provides ideal conditions conducive to disease development.
• Consider wide row and plant spacing. This increases ventilation between plants, reducing favorable conditions for plant infection.
• Use treated seed to avoid introducing the fungus to the field, such as Mertect 340-F, thiabendazole.
• Foliar applications of fungicides limit the rate of disease spread. Apply fungicides at first sign of disease and reapply according to the label if rainy weather is forecasted. Thorough coverage of the plant canopy is important. Options include Quadris (azoxystrobin), Endura (boscalid), or Headline (pyraclostrobin).

For more information, refer to the UC IPM website for dry beans or publication 8259, Ascochyta Blight of Garbanzos, by Frate et al. 2007.

Our cool, wet conditions are favoring white mold (sclerotinia) in garbanzo beans this year. This disease is characterized by a white fungal growth that attacks the above ground plant parts, often near the soil line for garbanzos. The affected tissue dries quickly and bleaches to a pale tan or almost white color.  Black sclerotia formed on the stem of dying plants are often visible. These are masses of fungi compacted to form small, dark pellets that range in size from 1-8 mm (about the tip of a pencil to a pencil-size eraser) that allow the fungus to survive in adverse conditions (e.g. hot, dry weather). When the main stem is affected near the soil line, the entire plant wilts and dies; lesser infections will show dead branches, causing yellow flagging in the field.

The black sclerotia pellets formed in the mold can survive many years in the soil and re-infect crops. The development of white mold is greatly influenced by prevailing weather conditions and certain agronomic practices such as late extensive irrigation, plant density, and plant growth characteristics, which are all closely linked with the life cycle of the pathogen. A thick dense plant growth provides a micro climate of cooler temperatures and high moisture beneath the plant canopy where conditions are favorable for disease.

The best control focuses on cultural practices to prevent the disease from developing in the field. This includes higher beds that keep lower plant parts and tops of the bed drier, wider row spacing to increase air movement in the canopy to keep the foliage drier, and planting garbanzo beans later (December through early February) rather than early (November) since the extra growing time can result in heavy canopies that increase humidity and cause infestations late winter/early spring (favorable disease conditions, 68^oF-77^oF). Crop rotation generally does not prevent white mold infection due to the broad host range of the pathogen.

Fungicides are available for white mold control, but getting good coverage and control is difficult because the inoculum is in the soil and the disease is often deep in the plant canopy. Fungicides would have to be applied when plants are smaller to provide good coverage and protection from this disease. For a list of fungicides available for white mold control in in garbanzos, see http://ipm.ucanr.edu/PMG/r52100111.html.

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).