Symptoms

 • When blossom blight is severe, it is easy to identify healthy alfalfa racemes compared to blossom blight racemes (Fig. 1). The blossoms hang in matted clumps covered by either grey, dusty growth (Botrytis cinerea) or white growth (Sclerotinia sclerotiorum) that eventually develops into hard black sclerotia.

 • Cool, wet weather promotes blossom blight development (Fig. 2). Cool wet conditions also result in reduced flowering, reduced pollinator activity, and increased foliar disease pressure. All of these reduce yield potential, but the only one that producers can do anything about is blossom blight.

 • Early symptoms in a field are difficult to identify and are easily confused with other problems, so symptoms of blossom blight are often overlooked or incorrectly identified. This is why the problem was not identified in western Canada until 1993  (Reference 1)

• Infected plants flower, but the bloom does not persist. In fields were pollination is adequate (high frequency of tripped flowers), but pod set is poor and injury is not due to insect pests, blossom blight in likely the cause. If left unchecked, blossom blight slows or stops pod set, and yield is reduced.

• An early symptom of blossom blight is that flowers clump around the stem, held together by fungal strands (Fig. 3). This occurs first in the lowest blossoms of the raceme. Death of infected flowers can occur even before the topmost flowers open.

Factors that promote blossom blight

• Blight tends to be most severe in portions of fields with a dense plant canopy (e.g. low spots) and in sheltered areas with reduced air movement.  (Reference 2)

• Outbreaks are associated with prolonged cool, wet weather during flowering. (Reference 1)

• Yield losses can be high (50-100%)  (Reference 1)

• Rainfall just before and during flowering is the most important factor that promotes development of blossom blight. If the weather becomes hot and dry, disease development in an infested field quickly stops. At temperatures over 30° C, disease development stops even under rainy conditions.

• Fields with severe blossom blight in one year do not develop blight the next year if conditions are dry during flowering.

• The pathogens that cause blossom blight can be carried in, on, or with seed. However, in the areas of western Canada where alfalfa is grown for seed, both pathogens are already solidly established on a wide range of crops. Therefore, seed testing and fungicide seed treatments to prevent introduction of these pathogens are not required.

• Early detection of infection is important for minimizing yield loss. Fungicide application to fields with severe infection often increases yield substantially, but yields are still low because many blooms have already been lost.

Cause

• Blossom blight is caused by two fungal pathogens, Botrytis cinerea and Sclerotinia sclerotiorum. (Reference 3)

• Botrytis cinerea can persist on dead material for many years, so its presence at low to moderate levels is normal. It also attacks other crops such as pea, bean, lentil, and chickpea. Infected leaves and stems become covered with a fine grey growth (spores) that gives it its common name ‘grey mold’.

• Sclerotinia sclerotiorum can persist in the soil as resting structures known as sclerotia, for many years. Air-borne spores infect flowers and other susceptible tissues, and then grow from there into the stems. Infected stems become, bleached, brittle, and covered in white fungal strands (Fig. 4), which give it the common name ‘white mold’. It can cause severe losses in canola and other broad-leaved crops.

Monitoring for disease

• Assessing fields for blossom blight should start shortly after the crop starts blooming (late June to early July) if conditions are cool and wet, and should be repeated at about 5-day intervals throughout mid bloom if conditions continue to be conducive for disease.

• To sample a field, initially assess sites that are particularly at risk of blossom blight, such as sites close to shelterbelts or in low spots in the field, to provide an early-warning of disease. If these high-risk sites show signs of disease, assess several sites that are more representative of the entire field.

• A quick and practical way to determine if blossom blight is causing damage in the field is to give the lowest flower(s) in a raceme a gentle pull. This should produce one of three results:

1) the flower comes off cleanly to uncover a developing pod (the result you want!),

2) the flower comes off cleanly by itself (not pollinated, or damaged by insects), or

3) the flower pulls off very easily and is matted together with other flowers by fungal strands (a symptom of blossom blight).

• As a research tool, selected growers were provided with test kits consisting of solid growth media in plastic Petri dishes, forceps to pull flowers from plants, and bleach to sterilize the forceps between samples. We provided a book of descriptions and pictures to help growers interpret what grew from the flowers onto the media (to request a copy of the Blossom Blight Test Kit, please email). The drawback for growers was that the sampling was time-consuming and strange, and results from sampling took five or more days to develop. Long before the fungi had grown and been identified, growers had already made a decision about fungicide application for disease control.

Risk of yield loss

• The risk of serious yield losses from blossom blight is related to two factors:1)  the frequency of infected flowers, and 2) weather. Moderate to high levels of flower infection (60 – 80%) followed by a period of cool, wet weather produce a high risk of serious yield loss.

• When deciding whether or not to apply fungicide, consider the weather forecast for the coming week. When conditions are hot and dry, blossom blight stops increasing. In fact, flowers that are already infected soon fall off the plant under dry conditions, so disease levels may decline quickly. When this happens, losses are small and a spray application does not increase yield.

• Studies are underway to develop a simple checklist that growers can use to assist them in deciding whether or not to apply a fungicide to a particular crop (Reference 4 and Reference 5). Information will be provided on this site as the system is developed and validated.

Managing Blossom Blight

• Early detection of moderate to high levels of infestation is critical for minimizing yield loss. When disease is present in a field and conditions are conducive for disease development (cool and wet), blossom blight incidence can increase quickly. Producers can dramatically increase yield under such circumstances by making a fungicide application before disease becomes severe.

 (Reference 6); (Reference 7) ; (Reference 8); (Reference 9); (Reference 10) ; (Reference 11) .

The most easily accessible are in the Pest Management Research Reports, but other sources include information used to support registration of fungicides for control of blossom blight, available through the Pest Management Regulatory Agency of Health Canada  and proprietary information developed by the Saskatchewan Alfalfa Seed Producers Association . Check provincial recommendations for registered products and rates of application.

• Small differences in disease reaction among cultivars have been identified (Reference 12).  At least some of these differences were related to differences in flower colour (Reference 13). However, these differences were too small to have an important effect under field conditions.

• Alfalfa is a perennial crop, so there is no opportunity to use crop rotation to reduce disease levels after the crop is established. Many (most) of the broad-leaved crops grown in the region are hosts for one or both of the pathogens that cause blossom blight, and both pathogens are endemic in fields, ditches and even grasslands and woodlands, so isolation of fields away from sources of infection is not possible and crop rotation would unlikely to have an important impact even if it were possible. Finally, alfalfa cultivars are all susceptible to blossom blight, and the quality of the alfalfa seed used to plant the field rarely if ever affects subsequent development of these diseases. Therefore, application of fungicides remains the only effective management option for blossom blight.

• Rotating fungicides to minimize the risk of pathogens developing insensitivity is a good standard practice for all crops and diseases. For blossom blight, where fungicides represent the only option, the crop may need to be treated over several years, and both pathogens are known to develop insensitivity to pathogens (Reference 14).  It is even more important that most. Consult provincial recommendations for fungicide rotation and application frequency.

Acknowledgement

The authors thank the Canada-Saskatchewan Agri-Food Innovation Fund, the Saskatchewan Alfalfa Seed Producers Association, the Agriculture Development Fund of the province of Saskatchewan, the Alberta Agriculture Research Initiative, the Canadian Seed Growers Association, and the Matching Investment Initiative of the Government of Canada for financial support. Also, special thanks to Wayne Goerzen, Gerald Huebner, and R. Linowski for their contributions to the project, to K. Bassendowski, B. Wong, and K. Anderson for technical assistance, and to all of the growers who participated in the study.