Introduction

This disease, caused by the fungus Fusarium oxysporum f.sp. radicis-cucumerinum, was first observed in Greece and the Netherlands. It was reported in British Columbia, Canada in 1994 and later in Ontario in 2000 causing 10% and 25%-35% losses, respectively.

Symptoms

Symptoms include wilting of plants at the fruit-bearing stage and during hot weather. There is yellowish or buff discolouration of the outer tissues of the crown of plants, however the white, cottony-growth known as fungus mycelium is not evident at this time. The fungus colonizes the cucumber stem beyond the visible disease symptoms. The advanced stages of the disease involve progressive upward colonization of the stem. A longitudinal cut of the crown shows the breakdown of cortical tissues (Figure 1.) and secondary infection by bacteria follows.

Figure 1. Stringy stem with light salmon colouration on the outside. Note the breakdown of the inner tissues and the white cottony-like growth of fungal mycelium on the surface of stem.

Figure 2. Note base of cucumber plant with buff or yellowish-orange colouration on stem due to mycelium of Fusarium oxysporum f.sp. radicis-cucumerinum.

Severely affected plants have a stringy stem with light-salmon or pinkish-orange spore masses and white fungal, cottony-like growth on the outside of the stem (Figure 2.). These plants turn brown and die, especially with high fruit loads and hot weather. The roots of affected plants remain firm in contrast to pythium root rot where the roots become completely rotten.

Cucumber and muskmelon are very susceptible. Watermelon is also susceptible while pumpkin and squash show only mild symptoms. Pepper and tomato do not show disease symptoms.

Epidemiology

The fungus can survive in plant debris and in soil for many years as chlamydospores (i.e. overwintering spores) and for shorter periods on greenhouse structures between crops as another type of spore known as conidia. It can spread in recirculating systems via the irrigation lines. Infection through root tips and wound sites occurring during transplanting is common. The fungus may also colonize rockwool blocks and may spread via root contact within rockwool slabs. Inoculum early in the growing season is important for disease development. Although numerous spores are produced on the diseased stem tissue, they are not readily dispersed aerially (only 1-2 m) because of the slimy material in which they are contained. Water dispersal is a more likely means of spread within the greenhouse and may cause contamination of the growing media. Other sources of spread in the greenhouse include the movement of infected plants, use of contaminated pruning instruments, or via clothing of workers who have come into contact with affected stems of plants.

Seed infection by Fusarium oxysporum f.sp. radicis-cucumerinum is suspected but has not yet been demonstrated. However, it has been reported for Fusarium oxysporum f.sp. radicis-lycopersici, a similar pathogen of tomato. Temperatures around 20°C favour infection of young plants especially during the first 4 weeks if plants are under physiological stress. Disease does not develop at 32°C, while older plants are less susceptible.

Control

Apply an integrated approach to disease management in the greenhouse. This includes resistance, prevention, and sanitation measures.

Resistance

Refer to seed dealers for listings of susceptible and resistant cultivars.

Prevention

Studies show that seed treatment can reduce low levels of seedborne infection if this is present. Refer to OMAFRA Publication 363: Vegetable Production Recommendations for available seed treatments.

Apply control measures before seeding and setting plants in the greenhouse since infection likely occurs during the first 4 weeks and older plants are less susceptible. Check seedlings for early death and discard those plants as well as healthy plants around the affected ones in the seedling trays. Early detection of the disease increases the chances of eradication. Check transplants carefully for symptoms such as wilting or stem infection. Use only plants that appear to be healthy as transplants. Bring suspect plants to an expert or plant disease clinic for diagnosis. Ensure all workers are aware of the disease symptoms and that they are instructed to alert the management at the first sign of these symptoms. Identify diseased plants with coloured tape to alert workers. Make sure workers are aware of the spread of spores if they touch the affected stems or growing media of affected plants with their hands or clothing. Work in the affected areas of a greenhouse last, after working in the areas where the disease has not been observed. Do not move carts and crates from infected to non-infected areas. Keep visitors out of affected areas.

Sanitation

Discard slabs, bags, cubes or other media that had infected plants growing in them previously. Do not replant into the same material unless it has been steam-sterilized. Remove all crop debris from affected greenhouses and strings from affected plants. Remove infected plants carefully taking care not to allow contact of affected portions of plants with adjacent plants and place them in a plastic bag. Discard the diseased material at a location away from the greenhouses to ensure that this fungus inoculum is not carried back into the greenhouses by workers, wind, on tires, and by insects such as shore flies and fungus gnats. Additionally, remove about 1-2 plants on either side of the plant(s) exhibiting symptoms and place in garbage bags. If the material is disposed of in a cull pile then ensure that the cull pile is located away from the greenhouse as far as possible. Cover the cull pile to prevent insects such as shore flies and fungus gnats from carrying the fungus spores back into the greenhouses. Alternatively, infected plant debris may be burnt or taken to a landfill. It should not be left in an open field or be incorporated into the soil in fields, particularly if other susceptible field crops such as muskmelon, watermelon, pumpkin or squash will be planted there later. After harvest, apply proper disinfection procedures in greenhouses where affected plants were located.

Maintain foot baths with adequate fresh disinfectant at every entrance to the greenhouse. Sanitize pruning tools used to handle infected plants by dipping in disinfectant after each contact with the affected plant. After leaving the affected areas, have workers, particularly those who come in contact with diseased plant material, discard disposable gloves and boots in a proper manner, leave shoes/boots for disinfection, and leave coveralls for laundering and disinfection.

Drip lines and drip stakes of affected plants should either be replaced or cleaned by removing deposits using an acid followed by use of a commercial disinfectant, and rinsing with water afterwards. Spores can survive in the lines or recirculating systems so disinfect irrigation lines at the end of the crop season. Flush the lines and tanks with a disinfectant several times over a 24 hr-period. Rinse afterwards with fresh water.

Biological

Refer to OMAFRA Publication 371: Growing Greenhouse Vegetables for available products.

Acknowledgments

The review of this Factsheet by Gillian Ferguson, OMAFRA is gratefully acknowledged. This Factsheet was prepared by Raymond Cerkauskas, Agriculture & Agri-Food Canada, Greenhouse & Processing Crops Research Centre, Harrow, Ontario.