Specialty Crops Disease Update – 2018
Environmental conditions often have strong influences on the occurrence of many diseases and their prevalence and distribution. This concept was readily illustrated for specialty crops in western Nebraska and certain diseases encountered during the 2017 growing season. The weather in 2017 was similar to that of both 2015 and 2016. Rainfall in spring and early summer was plentiful throughout the region, allowing good soil moisture for emerging crops. Cooler than normal temperatures were also present, which delayed crop development in sunflowers and dry beans. For example, several weeks in mid to late September and early October were cloudy and overcast with light rains and little sunshine.
This climate played a major role on the development of numerous diseases and other plant production problems during the season. Several thunderstorms with hail and tornadoes in June caused widespread damage to fields scattered throughout the Panhandle. This was followed by high levels of bacterial diseases (wilt, common blight) 7-10 days later. Dry bean and peas were the crops most severely affected. This report will summarize some of the major and unusual disease/pest occurrences encountered during 2017 for sugar beets, dry beans, sunflowers, potatoes, and dry yellow peas and other pulse crops.
Diseases of sugar beets in general in 2017 were not as severe as in recent years. However, due to the higher rainfall levels, Aphanomyces root rot was seen later in the year and at harvest. We continue to evaluate the integration of multiple methods for optimal management of Rhizoctonia root and crown rot. This is considered to be the most widespread and commonly occurring disease in Nebraska, and thus arguably the most important. Cercospora leaf spot was not a major problem this year, being sporadically found regionwide. It did appear in isolated areas later into September with the additional moisture from several rain events. The most severe damage from this disease is most often observed when night temperatures (midnight to 7 a.m.) exceed 70°F; however, in 2017 it was generally too cool at night for optimal disease development.
Dry beans in 2017 were strongly affected by several bacterial diseases. Multiple thunderstorms with high winds, hail and tornadoes in June caused widespread damage to fields scattered throughout the Panhandle. In many cases in Box Butte County, entire fields were destroyed and abandoned. Due to these storms, bacterial diseases were found a week to 10 days later. Bacterial wilt was particularly widespread, a disease we have not seen to this extent in several years. The cooler weather further resulted in widespread, white mold epidemics problems, late in the season, with one rare occurrence of a wet weather-oriented disease called gray mold.
Dry Yellow Peas
Interest in this crop continued to grow in 2017, with an estimated 70,000 acres planted. We also completed a three-year study to survey Nebraska production fields to determine the most prevalent and damaging diseases. Peas, like dry beans, were most commonly affected by a complex of several bacterial diseases. We are unsure of all members of this complex, but the brown spot pathogen, Pseudomonas syringae pv. Syringae, was most frequently identified. Several other bacterial pathogens found over the last three years are currently being characterized. We also identified low levels of root rots due to species of Rhizoctonia, Fusarium, and Pythium, but they are not major constraints with which to be concerned.
Cowpeas, also known as black-eyed peas, are another relatively new crop in Nebraska that show some promise and are gaining interest. Similar damage by bacterial diseases was observed from multiple fields in 2017 following thunderstorms. Bacterial wilt and common blight were the primary diseases seen. We also believe that this is the first documentation of bacterial wilt on this crop under natural field conditions and plan to publish about this early in 2018.
Chickpea production in Nebraska has been sporadic over the last 15 years due to a serious fungal disease called Ascochyta blight. Traditionally it has 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 named ‘New Hope’ with high levels of resistance was released in 2017 by Carlos Urrea.
The thunderstorms also affected sunflowers in Box Butte and Sheridan counties in 2017. We were alerted to some abiotic stresses observed in these locations that were a direct effect of the storms. The high winds caused lodging of plants in several fields. Two fields specifically were damaged by wind combined with the added problem of soil compaction. This made them more susceptible to falling over due to poor growth of severely restricted roots. The cooler weather was also responsible for scattered outbreaks of white mold and Phomospis 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 never before reported. It has not been named, but is a member of the family Tombusviridae, based on molecular and morphological characterizations. The hail storms also contributed to appearances of the Rhizopus head rot disease in affected areas of western Nebraska.
We saw no major problems to commercial potato production in 2017. From our trial plots we did identify both late blight and early blight (two common diseases). We continue to evaluate new fungicides for efficacy in managing these two fungal diseases. We also observed an insect-related problem called psyllid yellows. After the insect feeds on potato leaves, the tops become wilted, dry up, and die. This damage is due to a toxin that is inserted into leaves during the feeding process. We have encountered this problem the last several years in our potato trial plots, and it reappeared again in 2017.