New breakthrough regarding Fusarium in asparagus
Fusarium crown and root rot (FCRR) disease of asparagus causes wilt, fern chlorosis, vascular discoloration, root rot, and crown death significantly reducing yields.
Michigan is the largest producer of asparagus in the United States, providing spears both for processing and the fresh market. In spring, asparagus is direct seeded into a nursery and grown for one year. The resulting crowns are then transplanted into production fields and a limited number of harvests take place until the third year of growth. Michigan’s asparagus is harvested from May through June after which the fern develops. Fusarium crown and root rot (FCRR) disease of asparagus causes wilt, fern chlorosis, vascular discoloration, root rot, and crown death significantly reducing yields. Unfortunately, fusarium is a persistent and pervasive soil plant pathogen in Michigan’s asparagus growing region, making management difficult. Cultural and chemical control options are limited.
Michigan’s growers have been treating crowns
Fusarium spp. prefer poorly drained soils and humid climates. The pathogen may be disseminated by wind, rain splash, and soil movement and is resistant to harsh conditions, surviving on plant residues and in the soil for 20 years or more. Limiting disease caused by Fusarium spp. includes removing asparagus crop residues from fields and cleaning machinery between fields, managing weed hosts, minimising plant stress, and practising crop rotation. Treating crowns with fungicides before planting and fumigating crown nurseries and production fields are methods that have been used by Michigan growers in recent years.
New active ingredients to better manage purple spot
Purple spot on asparagus spears and ferns is also a significant problem for asparagus producers in Michigan. Purplish lesions may affect 60-90 % of the spears, rendering them unmarketable. The emergence of this disease occurred with the adoption of a no-till cultural system. Due to the sporadic nature of disease occurrence, fungicide cost, and lack of control with some fungicide spray programs, it was desirable to apply fungicides according to a disease forecaster. TOMCAST, a disease forecasting system derived from FAST, (forecast system for Alternaria solani on tomato), appeared promising in managing purple spot disease on asparagus. The fungicides chlorothalonil and mancozeb are commonly used to protect the fern from purple spot but products with new active ingredients are needed to maximise control.
Newer fungicides and disease forecaster TOMCAST
A greenhouse evaluation of biorationals and a fungicide plus a field evaluation of fungicides for control of Fusarium crown and root rot on seedlings were conducted by B.R. Harlan and M.K. Hausbeck from the Department of Plant, Soil and Microbial Sciences of the Michigan State University. An evaluation of fungicides for control of purple spot on the fern was also conducted. For the greenhouse evaluation, disease development was moderate with the untreated inoculated plants. Industry standard fludioxonil (Cannonball WP) was the only treatment that resulted in statistically healthier crowns compared to the untreated inoculated control. The field evaluation of fungicide for control of Fusarium root rot on seedlings is shown in Table 2, and the evaluation of fungicides for control of purple spot on the fern is shown in Table 3. It indicates that fungicides can limit disease caused by soilborne and foliar pathogens. The fungicides that were of primary interest include fludioxonil (Cannonball WP) applied as a drench to seedlings for control of Fusarium crown and root rot or as a premix of pydiflumetofen + fludioxonil (Miravis Prime SC) for purple spot foliar disease. Currently, these fungicides are not registered for use on asparagus seedlings or fern in the United States but could fill an important void in disease management. Fungicides containing chlorothalonil (Bravo WeatherStik SC) or mancozeb (Roper), “would be good candidates for use in an overall program that includes pydiflumetofen + fludioxonil (Miravis Prime SC),” the researchers said, later adding that they “can be used in conjunction with the disease forecaster TOMCAST to provide long lasting protection.” They also said that, “The asparagus crown nursery offers a unique opportunity to protect the developing crown from disease to ensure that plant propagules used to establish production fields are as healthy as possible.”
Suppression of Fusarium with nano micronutrients
Past work with chloride salts has shown that fertility is very important in delaying the effects of decline due to FCRR. A strong positive association was noted between chloride applications, micronutrient (Cu and Mn) uptake and disease suppression. In Quebec, scientists concluded that Fusarium abundance was negatively associated with Cu and that declining fields had reduced levels of Mn. More recently, B and Mn deficiency has been associated with asparagus decline. Applying micronutrients in nanoscale may offer a novel approach to deliver these elements to asparagus roots. The oxide forms are relatively non-toxic when compared to the ionic salts. The large surface areas allow for enhanced dissolution and their extremely small size, these particles can enter and move within plant tissues for intra-plant transport. Split root pot culture was used in the greenhouse assays to test the efficacy of nanoscale micronutrients such as B (500 nm), CuO (40 nm), MnO (30 nm), MoO (100 nm), and ZnO (10-30 nm). Field studies were also conducted and research plots were established in sites that had previously been planted to asparagus.
Cu and Mn associated with plant health
The first preliminary greenhouse study conducted by W.H. Elmer, N. Zuverza-Mena and J.C. White from the Connecticut Agricultural Experiment Station screened B, CuO, MnO, MoO, and ZnO in split root pots and assessed the disease severity (% root lesions) on the exposed and non-exposed sides. Inoculation with F. oxysporum f. sp. asparagi resulted in root lesions over 58-60% of the root system, but when plants were treated with CuO and MnO, the exposed and non-exposed root sides had significant reductions. Nano-Cu reduced disease severity from an average 59% to 23% on the exposed side and from 59% to 30% on the non-exposed roots, demonstrating a systemic response, the researchers said. Similarly, nano MnO reduced disease severity from 59% to 16% on the exposed side and from 59% to 28% on the non-exposed roots. Molybdenum oxide reduced disease on the exposed side, but not on the non-exposed side, while nano Zn was ineffective on the exposed, but systemically reduced disease on the non-exposed side. B did not affect the disease severity. These findings agree with reports in the literature that Cu and Mn were positively associated with plant health.
One crown soak to increased yield
Although most nano micronutrients improved growth and suppressed disease, nano CuO and nano MnO were superior in the control split root studies. Nano CuO and nano MnO were not systemic in the plant but were retained in the exposed treated root only. However, both nano CuO and nano MnO promoted a systemic defence to infection and colonisation of FOA in non-exposed roots. Field studies were also conducted by the same scientists in Hamden and Griswold. Yields from 2020 and 2021 were combined. All of the yield variables showed the same trend so only marketable spear yield was presented. Yields in untreated plots in Hamden were the lowest, but soaking crowns with B, CuO. MnO, MoO or ZnO in 2018 led to 1.5-, 1.8-, 1.9-, 2.0-, or 1.3-fold respective increases in marketable yield. Responses were not as striking in Griswold, but notable differences were still evident. Increases of 35%, 32%, 23%, and 30% in the trimmed spear weights were observed for B, CuO, MnO and ZnO respectively. “The observations that a single crown soak treatment at planting could lead to increases in yield three years later is astonishing,” the researchers said. While fumigation and fungicide soaks/drenches have resulted in improved health during the first year, the suppression did not last and/or was cost prohibitive. In contrast, nano forms of micronutrients were effective in suppressing disease and increasing yield after three years.
Effect of flooding period on asparagus growth
Asparagus has been promoted as a paddy field conversion crop in Japan. However, converted paddy fields are prone to excessive soil moisture due to their low permeability and high-water retention. In previous research it has been reported that asparagus grown in such conditions had rotting underground stems and roots, resulting in reduced yields. Furthermore, in recent years, torrential rains have caused flooding of asparagus fields in Japan, affecting the growth and yield of asparagus. However, mitigation measures for flooded fields have not yet been established. Poor field drainage contributes to the development of soil diseases, such as wilt and blight. Fusarium oxysporum f. sp. asparagi, for example, is widely distributed in production fields and causes extensive damage to asparagus production. Although F. oxysporum has such a serious impact on asparagus cultivation, there’s been scarce research into its relationship with flooding. Therefore, a study was conducted by T. Sonoda from Rakuno Gakuen University, Japan to determine the effects of different periods of flooding and F. oxysporum f. sp. asparagi infection on asparagus growth in order to contribute to the improvement of asparagus cultivation methods in former rice paddies.
How flooding length affects growth
The first experiment studied the effect of flooding-period length on the growth of asparagus cultivars. The main asparagus cultivars used in Japan, ‘UC157’ and ‘Gijnlim’, were used as test cultivars. One month after sowing, their seeds were transplanted to PVC tubes filled with horticultural medium (photo) and grown in a glasshouse. Ten days after transplanting, when the roots had grown 20 cm, water was poured to the ground level to flood the asparagus seedlings. Flooding periods were 0, 1, 5, and 10 days. After the applicable flooding period, the plots were drained of excess water and thereafter irrigated in the same way as the control plots. Seedlings were then removed and their maximum grass height, number of stems, maximum root length, degree of root damage, aboveground dry matter weight and belowground dry matter weight were assessed. Root damage was determined by using a five-level index to identify root necrosis and rotting.
Root lengths tended to be shorter with increasing durations of flooding, but the extent of shortening did not vary between cultivars. The degree of root damage increased with increasing periods of flooding. Differences in grass height and above- and below-ground dry matter weight were observed between the cultivars, and no differences were observed between them during the different flooding periods. “No differences in stem and root number were observed between different periods of flooding or between cultivars. There was no interaction between the duration of flooding and cultivar type for any of the growth parameters,” the authors said.
The importance of drainage
Prolonged flooding seems to inhibit root growth. But even if a field was flooded, the impact on asparagus would be minimal if rapid drainage could be achieved. “These findings suggest that when asparagus is grown in paddy-conversion fields, it is important to construct ditch drain or underdrain in the field to quickly drain excess soil moisture and avoid prolonged flooding of the root zone,” the researchers wrote. They also said that If these measures are not sufficient, “it is necessary to implement cultivation with raised rows to ensure drainage and sufficient rooting area and to use water-tolerant cultivars.”
The second experiment studied the effect of infection with F. oxysporum f. sp. asparagi and flooding-period length on asparagus growth. Growing conditions and methods for this experiment were performed as described in the first experiment. F. oxysporum f. sp. asparagi was used for inoculation, which was isolated from asparagus production fields in Japan. Root damage observed on plants inoculated and flooded for 5 and 10 days was higher than plants inoculated and not flooded. Further, statistical test results indicate that there is an interaction between presence of fungus and duration of flooding. In F. oxysporum-inoculated areas, root length was significantly longer in the 0-day flooded area than in the 5- and 10-day flooded areas, and the degree of root damage increased as the flooding-period length increased. In the absence of F. oxysporum, there was no difference in root damage depending on the duration of flooding, while in the presence of F. oxysporum, the longer the duration of flooding, the greater the root damage. The results of this study suggest that disease development caused by F. oxysporum is accelerated by prolonged flooding. “Adding drainage measures to disease-resistant cultivars would reduce the impact of flooding and F. oxysporum f. sp. asparagi on asparagus growth,” the author said.
