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New fungicide to help control of Mycosphaerella

Commonly known as ‘Myco’, the fungus Mycosphaerella melonis causes black stem rot, or gummy stem blight, on cucumbers and other cucurbits. In the past outbreaks of the disease have caused economic damage, but it has not been widespread in the UK, however anecdotal evidence suggests that it is become more widespread.

“We have definitely seen more in recent years, although this may partly be due to the varieties we are growing and moves to reduce chemical use,” says Jo Cappalonga who grows around 2.5 ha of cucumbers in Essex. “Mycosphaerella can be a particular issue during periods of dull weather. We are not sure exactly what type of conditions it favours, but it seems to be less of a problem at the height of summer when there is plenty of light and heat. For example we saw the disease last year when we had ‘the beast from the east’ at the end of February.” Having said that, Jo feels that there is a tendency for the disease to develop towards the end of the season. “There also seems to be a variety effect, with varieties which are tolerant to mildew appearing to be the hardest hit by Mycosphaerella, particularly in the autumn,” he adds.

The fungus can cause extensive stem and leaf infections which in the worst cases can result in the death of severely infected plants, and while infection of the flowers can lead to rotting fruit under favourable conditions, work funded by AHDB Horticulture suggests that latent fruit infection which can be hard to spot externally but which nevertheless results in rejected fruit, continues to have a significant economic impact.

Jo also points out that there is an element of ‘catch-22’ for growers when it comes to the disease. “As we are trying to avoid spraying chemicals for mildew control we may choose varieties which are more mildew tolerant, but this tends to result in a greater incidence of Mycosphaerella,” he points out. “Conversely, if growing varieties which are more susceptible to mildew, not only may they be less susceptible to Mycosphaerella, but you will also spraying to control mildew which will also control Mycosphaerella. There are still a lot of unknowns about the disease, such as what causes it to develop. There is a need for a lot more research and work to understand the disease.”

In recent years chemical control has relied on the use of SDHI fungicides, a less than ideal situation for both control and resistance management. “A number of these SDHI’s are not fully compatible with the use of macro-biologicals or with some of the more recently established biopesticides,” points out Keith Sellars of Hutchinson’s. “Growers often tend to prioritise Mycosphaerella control, and subsequently secondary pests or pathogens can sneak into the crop.”

The recent off-label EAMU approval of Frupica (mepanipyrim) from Certis for the control of powdery mildew and botrytis on cucumbers means that growers now have a new management tool available. “The availability of Frupica is a really useful option, providing an alternative active for a robust anti-resistance strategy,” adds Keith. “It also has the added benefit of low impact on macro and micro biologicals.”

Selchuk Kurtev, IPM Manager at Certis, point out that while Frupica may be new to UK cucumber growers, it has been used successfully for Mycosphaerella control in The Netherlands for a number of years. “In the past Mycosphaerella was not really considered an issue in the UK, but it has caused problems for growers elsewhere,” he point out. “With Mycosphaerella recently beginning to cause trouble for growers in the UK, it is good they now have access to Frupica which is now one of the key products used for Mycosphaerella control by Dutch growers.”

Keith points out that this experience should reassure British growers who are considering the use of Frupica. “It’s reassuring when working with manufacturers who have conducted crucial research at a European level for actives on such minor crops and have a real understanding of the pathogens we are dealing with,” he stresses.