Management Strategies for Fusarium Wilt in Spinach
Fusarium wilt, caused by the fungus Fusarium oxysporum f. sp. spinaciae, has been reported in all spinach growing areas of the world including: Canada, U.S., Sweden, and Japan. This soilborne fungal disease can cause up to 100% crop loss. As the disease progresses, symptoms include: stunting, wilting of the older leaves, necrosis, and plant death (Figure 1). The vascular tissue of the root turns from white to dark brown as disease progresses (Figure 2). The fungus invades the root and blocks the vascular system reducing the plant's ability to uptake water. Wilting is most severe during the hottest part of the day and moderately infected plants may recover overnight. Fusarium wilt of spinach occurs annually, most aggressively in summer months when soil temperatures are 25-30°C. Severe symptoms are most often found in full-sized spinach than in baby spinach production as the disease takes time to spread within the root; however, Fusarium oxysporum can also cause damping-off in spinach seedlings.
Figure 1: Progression of Fusarium wilt on spinach leaves.
Figure 2: Progression of disease in spinach roots: left - clean roots; centre - early symptoms; right - advanced symptoms.
This pathogen is spread throughout fields by farm equipment, people, and water. It can be introduced to farms on infested seed as this pathogen is seedborne. Once the disease has been introduced to a field it is almost impossible to eradicate, as soilborne pathogens are extremely persistent. Crop rotation is impractical for most growers as the long-term survival structure of Fusarium can survive for up to 15 years without a host. Currently, there are no effective registered products or resistant cultivars for the control of Fusarium wilt in Ontario spinach production.
During the summer of 2012, field trials were completed by the University of Guelph to identify possible control methods for managing Fusarium wilt (Figure 3). The trials included screening commercial varieties for susceptibility, efficacy of organic and conventional fungicides, and applications of limestone to increase soil pH.
Figure 3: 2012 field trials completed by the University of Guelph to identify possible methods for managing Fusarium wilt in spinach.
The study compared 25 cultivars from Alf Christianson Seed Co., Pop Vriends Seeds, Rogers-Syngenta Seeds, and Seminis Vegetable Seeds. The varieties C2606, Sardinia, POH-6116, and Carmel were found to be less susceptible while Unipack 12, Norgreen HF, Seven R, Tasman, and Falcon were found to be the most susceptible to Fusarium wilt. No resistant cultivars were identified as all varieties displayed some symptoms. The varieties Imperial Star and Persius were not suitable for the climate of southern Ontario in July as the plants bolted prematurely and were rendered unmarketable.
A total of six organic biofungicide products were tested for the control of Fusarium wilt. Two products, composed of Streptomyces lydicus and extracts from quinoa plants, suppressed disease severity equivalent to the best conventional fungicides by reducing disease severity by approximately 45%. The following biological active ingredients were ineffective at suppressing the disease: Bacillus subtilis, Gliocladium catenulatum, Coniothyrium minitans, and Streptomyces griseoviridis.
Fusarium wilt is favoured by acidic soils and soils around a pH of 8 tend to suppress disease. Dolomitic limestone (0, 10, 20 tonnes/ha) was applied to a commercial spinach field in the spring of 2012. The initial pH of the soil was 7.1 and over the course of the trial the pH remained unchanged for all treatments. A decrease in disease severity was found with a lime application of 20 tonnes/ha. As the pH was unaffected, the reduction in disease severity might be attributed to the increased calcium levels supplied to the crop. Calcium is known to play an important role in host resistance. Research from Washington State University found that with a soil pH of 5.5 limestone applications of 4 tonnes/ha reduced wilting incidence by 45%. Ontario soils with a pH less than 7 may benefit more from limestone applications.
This research is continuing in 2013, and will include additional biological controls as well as the biofumigant product Must-Gro. The results of this study are preliminary and are only based on one year of research. This project was also completed on mineral soil and so these results are not necessarily applicable to other soil types. Not all spinach cultivars tested in the trial are commercially available. Biological fungicides used in this study are not yet registered for use on spinach and should not be used. For pest control products registered on spinach in Ontario refer to OMAF and MRA Publication 838.
Organic growers can reduce the impact of Fusarium wilt of spinach by choosing less susceptible cultivars, selecting clean sites, and potentially the application of lime. Growers should monitor their fields for symptoms on a regular basis and check the roots of wilted plants. If severe symptoms have been identified in particular fields, attempt to avoid cropping these locations during summer months. The identification and registration of effective biological fungicides would improve integrated management of this disease.
Funding for this research was provided by the HQP Scholarship Program of the OMAF and MRA/University of Guelph Partnership and Collins Farm Produce.
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