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Keep seasonal leaf diseases in check

Advisory Blog
17.11.2020

Cool, damp conditions at this time of year increase the risk from a range of foliar diseases including rusts, leaf spots and powdery and downy mildews – whether on crops such as pansy and viola, cyclamen or primula; or on stock being overwintered, in unheated tunnels for example.

One that’s particularly damaging and difficult to manage is Botrytis, which is why I’m having a look at it in this first blog on the disease issues you need to be on top of through autumn and winter.

Sound cultural practice is the first line of defence for them all. Adequate ventilation and air movement will avoid zones of high humidity, which favour foliar diseases, building up in glasshouses or polytunnels. You may want to consider installing fans if you haven’t already – even with the vents open it is surprising just how little air movement there can be over quite large areas, especially at floor level.

Good plant spacing helps too, especially in crops with relatively dense arrangements of foliage where diseases like Botrytis can get established thanks to the humid conditions within that canopy.

Make it a routine to regularly pick over the crop during inspections, removing senescing or fallen flowers or foliage which can be a source of disease spores.

Most of these foliar diseases depend on the leaves being wet for a period so their spores can germinate. If using overhead irrigation, morning applications give leaves time to dry so the crop isn’t sitting wet overnight. Botrytis spores, for example, need prolonged leaf wetness and relative humidity above 95% for around three hours; given those conditions, they can germinate at temperatures as low as 10°C.

 

Botrytis: biology and symptoms

The pathogen Botrytis cinerea, commonly called grey mould, has a wide host range. It spreads by spores in the air or splashed by water but also produces resting spores, known as sclerotia, on, for example, infected plant debris.

Soft growth and stressed plants are particularly susceptible to infections and damaged tissue is a common entry point, which is why it’s worth avoiding damage caused by potting, handling or scorch from, for example, fertiliser application or crop protection sprays.

It’s the fungus’s ability to survive in a plant without any sign that makes it so difficult to manage. Latent infections can lie dormant for several weeks before symptoms appear, often triggered by some form of environmental stress.

The most distinctive symptom is the characteristic white fluffy mould, which turns grey, on leaves, stems and flowers. On bedding and pot plants, it will often be seen first on stem bases or on flowers, or anywhere the foliage is dense or in contact with growing media. Cyclamen flowers are especially vulnerable, but the initial symptom here is watery spots.

On woodier species, such as heathers, hebe, ivy, lavender and rhododendron, which are prone to Botrytis in unheated greenhouses, symptoms include browning and dieback of shoots, and scorching of foliage, before the typical grey mould appears.

 

Preventive spray programme

Once Botrytis is established in plant tissue, control using fungicides is difficult to achieve. Instead we’re looking for a protectant programme, which should start from soon after potting on the most susceptible crops. Good coverage on crops such as primula, where some leaves are close to or touching the growing medium, is particularly important.

Two of our broad-spectrum fungicides can form the core of a good protectant programme: Switch (cyprodinil + fludioxonil), which has outstanding Botrytis control and on-label approval for ornamental plant production; and Amistar (azoxystrobin), which can be used under EAMUs. Amistar’s translaminar movement is useful when it’s hard to target the undersides of leaves with a sprayer.

Botrytis is known to be capable of developing fungicide resistance but the three different mode of action groups that Switch and Amistar have between them will help with resistance management (see my earlier blog for more about managing resistance). You should alternate these, however, with at least one other active substance from a different group again. Effective biological protectants are also available and introduce a further mode of action to give you options for a programme with spray intervals of 10-14 days, which is usually adequate, without over-relying on any particular chemistry.

A broad-spectrum programme such as this protects against other foliar diseases like leaf spots and powdery mildews too.

In pack crops, keeping the plants compact with a growth regulator such as Bonzi has the knock-on benefit of helping to improve air movement within the crop.

For more information on managing resistance in Botrytis, take a look at our resistance management tools here.

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