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.

The annual Delta field meeting will be held on Tuesday, October 4th starting at 10:00am. This year, the agenda includes topics on both field corn and sorghum. The meeting location is at the UCCE field corn variety trial plot off of Brunk Road on Tyler Island. Please see the attached flyer for more information. The agenda is as follows:

Agenda

10:00am        Field corn variety evaluation – preliminary results

10:15am        Variety traits for the Delta 

10:30am        Sorghum seeding rates for optimum productivity – preliminary results

10:45am        Viewing of field plots

We have requested CCA continuing education credits; light refreshments will be provided. Please contact Michelle Leinfelder-Miles, Delta farm advisor (209-953-6100) with questions.

 

Please see this article on the UC Rice Blog for an update on armyworm populations. We will continue to monitor traps in the Delta through mid-September, and we will plan to monitor populations next season.

A land manager recently contacted me with questions about overseeding alfalfa. Her alfalfa stand is diminishing but not to the point of giving up on it. Rachael Long, Yolo County farm advisor, Dan Putnam, UC Davis alfalfa specialist, and Mick Canevari, San Joaquin County farm advisor emeritus, presented on this topic at the Alfalfa and Forage Symposium a few years back and wrote up a proceedings paper on this topic. Additionally, there is a production manual which is available through the UC ANR catalog.

The table below names seeding rates for various annual and perennial species that could be overseeded into alfalfa. The rates have a range (e.g. 4-8 lbs/acre). First, consider the field conditions and the alfalfa stand in order to pinpoint a rate in that range. For example, if the alfalfa is sparse, 2, then perhaps aim on the high end of that range.

Table. Seeding dates and rates for crops overseeded into alfalfa for the Sacramento and San Joaquin Valleys. In the Intermountain Region (Northern California) annual grasses are planted February to April; perennial grasses are planted August to September or March to April. (Table courtesy Long, Putnam, and Canevari, 2003)

If two overseeded species are desired (e.g. one grass and one legume), then the seeding rate could be reduced by 25-30% for both species. So, for example, if orchardgrass and red clover are to be planted over a sparse alfalfa field, then instead of planting the high rates of both, 8 and 12 lbs/acre, respectively, the rates could be reduce 25% to 6 and 9 lbs/acre, respectively, when planting both species.

The reason it is not recommended to overseed with more than two species is because some of the species will outcompete others. The ryegrasses (annual and Italian) are very vigorous and could outcompete red clover, for example. Orchardgrass or tall fescue are best paired with red clover. Berseem clover and a ryegrass, however, could work well together because they have similar vigor and growth timing.

Overseeding should take place in the fall, and ground preparation is important. Scratch up the soil with a harrow; then broadcast the seed, and roll it to slightly cover the seed. Keep in mind that with broadcasting equipment, if seed size is different, the heavy seed may settle to the bottom, and it may not result in a uniform planting of both species. Ensure that seed is mixed well before broadcasting, or use a grain drill. If substantial rain isn't on the forecast, it is best to irrigate the newly overseeded crop to ensure a good stand.

With proper considerations, overseeding could extend the life of an alfalfa field by 2-3 years.