Fusarium Stem and Fruit Rot of Greenhouse Pepper
Table of Contents
This disease was reported from pepper in commercial greenhouses in Ontario and British Columbia, Canada in 1991. Losses in fruit yield and plants were approximately 5%. Fusarium solani can attack a wide variety of plants including most greenhouse vegetables. Many physiologic races adapted to specific hosts have been recognized.
Soft, dark brown or black cankers are formed on the stem, usually at nodes or wound sites (Figure 1). These may girdle the stem in later stages of disease development. There is a dark brown discolouration of the internal portion of the stem that may extend a considerable distance (Figure 2). The lesions may eventually develop cinnamon or light orange-coloured, very small (<1 mm diameter), flask-shaped fruiting structures known as perithecia, which are the fruiting bodies of the fungus (Figure 3).
Figure 1. Black cankers on the pepper stem due to Fusarium solani.
Figure 2. External and internal dark brown discolouration of pepper stems due to Fusarium solani.
Figure 3. Light orange-coloured fruiting structures known as perithecia, which represent the secual stage of Fusarium solani, on a pepper stem.
Salmon-white cottony-like growth representing the imperfect stage of the fungus and known as fungus mycelium may also be present on the surface of stem cankers in late stages of disease development (Figure 4). Stem cankers restrict the upward flow of water resulting in wilting (Figure 5) and death of the plant. Pepper fruits may also develop black, water-soaked lesions beginning around the calyx (Figure 6). The lesions grow, coalesce and spread down the sides of the fruit. Copious mycelial growth of the pathogen occurs under humid conditions particularly when temperatures exceed 25°C. The leaves of affected plants may appear mottled, similar to magnesium deficiency. Early stem symptoms of this disease are very similar to symptoms incited by Erwinia carotovora subspecies carotovora which causes bacterial stem and peduncle canker in pepper (Figures 7, 8).
Figure 4. Note the presence of both the perfect stage containing perithecia and the imperfect stage witht he white cottony-like fungal growth on the pepper stem.
Figure 5. Pepper plant with wilting symptoms due to Fursarium solani.
Figure 6. Black lesions around the calyx of the pepper fruit.
Figure 7. Internal and external stem symptoms due to Erwinia carotovara subsp. cartovara, a bacterial pathogen of greenhouse peppers, are similar to symptoms incited by Fusarium solani.
Figure 8. Black stem canker due to Erwinia carotovora subsp. carotovara on greenhouse pepper. Note similarity of symptoms with cankers due to Fusarium solani in Figure 1.
Figure 9. Cinnamon-orange perithecium representing the perfect fruiting stage of F. solani with ascospores contained in many sac-like asci.
There are 2 stages in the growth cycle of the fungus and both of these occur concurrently on pepper stem/crown tissues. The perfect stage (Nectria haematococca) is where sexual recombination occurs and fungus spores, called ascospores, are produced in the cinnamon-coloured perithecium (Figure 9). This flask-shaped fruiting structure will arise during later stages of disease development on the stem under conditions of high humidity. Research from British Columbia found that the spores are forcibly ejected about 1-2 m from the perithecium at night. This is the primary means of natural dissemination in British Columbia greenhouses. Spore release at night is more favourable for disease development since periods of high relative humidity and even dew occur during this time. The other spores, called conidia, are produced asexually and in large numbers during the imperfect stage (Fusarium solani) They are not ejected but transmitted passively, and therefore are not as important in the natural dissemination in the greenhouse. These spores may be dispersed by water splash, on pruning knives and other tools, on clothing or on workers' hands.
The ascospore germination occurs during prolonged periods of high humidity (ie. greater than 95%). Greenhouse experiments indicate that periods with relative humidity ³ 90% in the greenhouse are actually higher at the leaf surface and therefore favour Nectria haematococca ascospore germination. Rapid or late (after the sunrise) temperature increase in the morning can result in dew formation and good conditions for ascospore germination because the dew point temperature exceeds the fruit and stem temperature. A slow temperature increase early in the morning of 1°C per hour ensures that fruit and stem temperatures reach daytime targets before sunrise. Also, if the greenhouse has restricted ventilation and poor drainage, this may create a "wet" climate that N. haematococca can exploit for ascospore germination. Perithecia present on rockwool blocks and on fruit lesions provide further aerial inoculum which, when accompanied by a "wet" greenhouse climate, result in numerous fruit and stem infections. Other factors, such as over-watering of the rockwool blocks, can stress the plants through oxygen depletion, resulting in a high incidence of crown lesions. Therefore, aggressive greenhouse climate management which avoids extended periods of high relative humidity, and precisely adjusted drip irrigation that avoids excessive wetness of the rockwool blocks, may reduce disease incidence even if the fungus is present on the blocks from the beginning of the growing season.
Ascospores on plant surfaces may survive several days under unfavourable climate conditions until free moisture or a nearly saturated environment is available for infection. However, the spores will not survive a crop-free period of 3-6 weeks. Hot days with fluctuating relative humidity are detrimental for ascospore survival.
Fusarium solani is extremely common in soils of Canada and is a saprohytic fungus, which means it can colonize dead or dying plant tissues. It can produce certain overwintering spores called chlamydospores that may remain viable for years. The fungus can invade pepper stems at the nodes or at the soil line, taking advantage of wounds created by pruning or salt damage. Rapidly growing, succulent crops are the most susceptible, as are ripening fruit compared to green fruit. Fruit that is damaged, especially around the calyx, is very susceptible to infection and rotting can continue in storage. Healthy, undamaged fruit is not usually attacked. The fungus may colonize fallen or aborted fruit and senescent flowers.
Growers in British Columbia have noted the occurrence of fruiting bodies of the fungus on rockwool blocks that are introduced into the greenhouse after the propagation process and during the early part of the growth stage of the plant. The rockwool blocks may allow the fungus to survive periods of unfavourable climate in the greenhouse throughout the growing season. When conditions are favourable for spore release and infection of the pepper plants then the ascospores could be released from the fruiting structures on the rockwool blocks. Since the rockwool blocks are constantly moist, this creates ideal conditions for release of spores even if the greenhouse climate may not be as favourable.
The pathogen can also be introduced accidentally via tools and equipment carrying diseased debris from adjacent greenhouses. Workers can transfer spores via their shoes and clothes when moving from a diseased area to an area free of disease symptoms or to another greenhouse.
Infection without symptom development (latent infection) may occur on crown tissue with symptom development evident as much as 2 to 3 months later or at the end of the season. Symptom development will be triggered by plant stress arising from a heavy fruit load, adverse environmental conditions, or senescence.
Include cultural practices, prevention, sanitation, environmental, and biological control measures in an integrated disease management program in the greenhouse for this problem.
Good crop hygiene and pruning by clean cutting will help to control this disease. Because the fungus has a saprophytic stage, it may easily colonize dead fruit, flowers or leaves and later form fruiting bodies that sporulate on this colonized tissue. Therefore, it is very important to remove pepper debris from greenhouse alleyways if the disease has been observed in the particular greenhouse.
Refer to OMAFRA Publication 836: Growing Greenhouse Vegetables for biological control recommendations.
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