Specialty Crops Disease Update January 10, 2019
The occurrence and distribution of plant pathogens are long known to be strongly influenced by the environment. We see evidence of this concept every season on specialty crops in western Nebraska, and 2018 was no exception.
Similar to 2015-2018, rainfall in spring and early summer was plentiful throughout the region, allowing good soil moisture for emerging crops. In fact, rain accumulations equaled or exceeded yearly averages in many locations. Rainfall totals ranged from 12 to 20+ inches between May and August, depending on location. Temperatures were lower than normal during the summer, with few days, if any, exceeding 100°F. This type of weather often will delay crop development, but it also promotes the incidence and severity of certain diseases that we do not encounter each year.
This climate played a major role in the development of several diseases such as root rots, rust, downy mildew, and white mold. Thunderstorms with hail caused widespread damage to fields scattered throughout the Panhandle, but still very localized. Some fields were completely destroyed with others left undamaged. Typically, the crop most severely affected by these storms are dry beans and peas due to a complex of bacterial diseases (common blight, brown spot, wilt) and white mold. This report will summarize some of the major and unusual disease/pest occurrences encountered during 2018 for sugar beets, dry beans and other relatively new pulse crops, sunflowers, potatoes, and other minor specialty crops.
Root rot diseases of sugar beets in general were higher in 2018, due to optimal environmental conditions. However, we continue to evaluate the integration of multiple methods for optimal management of Rhizoctonia root and crown rot, as it is the most important and consistently damaging disease in Nebraska. The dry rot canker disease was again documented from several fields in the older growing areas of the Platte River Valley.
Furthermore, a number of seldom-seen root diseases, such as Fusarium wilt and Aphanomyces root rot were found more commonly than in previous years. Cercospora leaf spot was sporadically found region-wide, but it did appear in isolated areas in late August and early September. Although generally not problematic, bacterial leaf spot looks menacing with large, very conspicuous lesions on lower leaves. It was much more prevalent this season due to the cooler, wet conditions.
Dry beans in 2018 were strongly affected by several bacterial diseases. Multiple thunderstorms with high winds and hail in June caused widespread damage to fields scattered throughout the Panhandle. In many cases in Box Butte County, entire fields were destroyed and abandoned. As a result of these storms, bacterial diseases were often found a week to 10 days later. Bacterial wilt, common blight, and brown spot were readily found in affected fields. The cooler weather further resulted in widespread, but isolated white mold epidemics problems.
Dry Yellow Peas
Interest in this crop continued to grow in 2018, with another estimated crop of approximately 70,000 acres planted. As a result of a three-year study to survey Nebraska production fields, we found that a complex of bacterial diseases were the most commonly identified disease problem, similar to what we see in dry beans. While research is ongoing to identify all members of this complex, we do know that the brown spot pathogen, Pseudomonas syringae pv. Syringae, is a member. We also have documented several root rots caused by unidentified species of Rhizoctonia, Fusarium, and Pythium, but they are not major problems to be concerned about at this point.
Cowpeas, also known as black-eyed peas, are another new pulse crop in Nebraska that shows some promise, and interest in its cultivation is continuing by some producers. Damage due to bacterial diseases similar to that from dry beans was observed from multiple fields in both 2017 and 2018 following thunderstorms. Bacterial wilt and common blight were found in all fields. This is the first documentation of bacterial wilt on cowpeas from natural field conditions in North America, and isolates have been collected and stored from multiple fields in both years. Studies are ongoing to formally confirm identity, pathogenicity, and virulence on both cowpeas and dry beans.
We also found very low levels of an unidentified virus within research plots in 2018, likely to be seedborne. It is only of academic interest with little cause for concern in the future. We also found high levels of white mold in plots (12-15% incidence), which is a new finding for this region, but not surprising given the weather and fact that the cowpea is a reported host of the white mold pathogen in the greenhouse. Lastly we also observed wilting symptoms associated with a root rot that was randomly scattered throughout plots. We have tentatively identified a species of Fusarium as the causal agent, but further studies are ongoing.
Chickpea production in Nebraska has been sporadic over the last 15 years due to a serious fungal disease called Ascochyta blight. It has traditionally been the primary limiting factor and we continue to study new methods for its management, including new fungicides and developing new cultivars with better disease tolerance. After more than 10 years of research, a new cultivar with high levels of resistance was released in 2017 by Carlos Urrea, dry bean breeding specialist at the university’s Panhandle Research and Extension Center in Scottsbluff. It’s named ‘New Hope.’ The cool wet weather in 2018 was ideal for blight disease development. As a result, we were able to further test new chickpea germplasm sources for resistance in the process of creating additional high yielding, resistant cultivars for Nebraska growers.
The environmental conditions were instrumental in the appearance of white mold in several fields from Box Butte County. This is remarkable as I have never before seen this disease in chickpea. Chickpea stands rarely close rows completely. However, when they do (promoted by the weather conditions in 2018), a micro-climate is created with higher humidity, cooler temperatures, and less wind movement through plant canopies. All of these factors favor both white mold and A. blight.
The cooler weather was also responsible for delayed senescence of sunflower crops, which may have harmed some fields due to an early freeze. Some crops which had not completely matured may have been damaged. We saw the first hard freeze with 6-8 inches of snow throughout western Nebraska in the first week of October. These unusual conditions also resulted in scattered outbreaks of white mold and Phomopsis stem canker, the latter of which is becoming a major threat throughout sunflower growing areas of North and South Dakota.
We also have now tentatively identified a novel virus disease from Nebraska that is apparently new to science and not previously reported. It has not been named, but is a member of the family Tombusviridae, based on molecular and morphological characterizations. This soilborne virus was again identified from volunteer sunflowers within a chickpea research field due to the cool rains early in the spring. It was also observed from dozens of early-planted ornamental sunflowers used as a border for other research plots. These same conditions also allowed the unusual appearance of downy mildew and the early stages of rust on the sunflower volunteers in early spring.
We received no samples or reports from potato production fields in 2018. However, from our trial plots we did identify Rhizoctonia rot on roots and stolons. Scab was also seen on certain varieties, but was not widespread or harmful throughout the entire area. The cool, very wet conditions also resulted in some late blight.
This is a common disease under these circumstances, but not always seen in western Nebraska. We continue to evaluate new fungicides for efficacy in managing these diseases for various chemical companies. Curiously, we did not find the insect-related dilemma called psyllid yellows. This is a malady caused by a toxin transmitted by the potato psyllid during the feeding process. We have encountered this problem over the last half decade in our potato trial plots, but it was mysteriously absent in 2018.
Another new potentially profitable crop for western Nebraska is mint, grown for the oil. Both spearmint and peppermint varieties/breeding lines are being evaluated at the Panhandle REC in small research plots. We observed a rust disease this year, again likely due to the cool and moist environment. It was observed only from several spearmint plots, but not throughout the entire study. We will be monitoring the plots of this perennial crop to study the pathogen’s life cycle and spore stages this winter and next spring. This is a new finding for us since mint cultivation is only in an experimental stage.