Pest Watch: Know your enemy... Broad bean chocolate spot

The best way to prevent its invasion is to create inhospitable conditions so understanding its lifecycle is an important step in combating its defoliating effects.

Ecology and Lifecycle

Chocolate spot is caused by not one but two fungi from the Botrytis family. The most common is Botrytis fabae (named for its host) and this fungal nasty only affects broad beans. However, Botrytis cinerea, the fungi that usually causes grey mould on many plants can also cause very similar symptoms.

Chocolate spot can be very mild, hardly affecting yields or extremely damaging, causing entire plants to collapse. What controls its voracity is the weather with cool, damp, overcrowded conditions providing the ideal temperature and humidity. It can pop up from late winter on autumn sown beans through to May on spring-sown crops. Luckily it does not infect any other common allotment bean crops but if you are a fan of using green manures it is worth knowing that it can survive in and infect vetch and winter field beans (actually also Vicia faba, basically identical to cultivated broad beans themselves).

When the fungus has ideal wet conditions it produces masses of airborne spores specially designed for dispersal. However, even when life is not treating it so well, this canny fungi can also produce much more durable overwintering structures called sclerotia which can linger in dead plant tissue. If weather turns dry after an initial infection it may completely halt the spread, keeping the pathogen contained in the round chocolate coloured spots but given wetter weather again these will rapidly expand.

Problems and Symptoms

Beginning as a delicate ‘dusting of cocoa’ type spotting, this fungus starts to cause dark, round, chocolate coloured spots on all parts of the plant. Although they start small, under favourable conditions these spots expand aggressively, merging with each other and coalescing to form large blackened necrotic areas. This causes leaves to shrivel and stems to collapse. Even with a minor infection the damage to the leaves photosynthetic apparatus can cause reduced vigour and pod yields.

Once present it is a tough disease to shake as the spores are tenacious but the intensity of infection can vary dramatically year on year as it is so much affected by weather conditions, the worst being when late frosts are followed by a mild and humid early summer. With the right conditions chocolate spot can spread very fast so be vigilant during mild humid weather.

Prevention and Control

The best strategy to keep chocolate spot at bay is to create freely ventilated warm conditions around your plants. Maximise the air flow by using slightly larger spacings between plants and staking and tying in rows so they do not flop in to each other. Improve drainage in your soil through good cultivation and the addition of organic material or even grit if needed and avoid planting in damp, humid or shaded sites.

Soils which are poor in potassium produce more vulnerable plants so enriching your soil with wood ash or seaweed should help but avoid nitrogen fertilizers as these produce tender, susceptible growth. A crop rotation is obviously key to avoid growing broad beans on the same site year after year. Timing can also help as broad beans planted for overwintering are more prone to infection so if chocolate spot plagues you every year, try early spring sowings in February instead.

Destroy all infected plant material as soon as you can during or at the end of the season and do take care to pick up the many stray leaves which fall from these plants. Most reinfection occurs through spores overwintering on dead plant material and may even occur on saved seed so it is a good plan to avoid saving from infected plants. That said if it is a precious heirloom variety then the seed will still be perfectly viable and all affected plants will still produce edible beans if pods set. The benefits of digging in broad bean roots for their nitrogen containing nodules are well known. Even with a bad infection you can still do this as long as you cut away all the old stems to just below ground level in case these harbour spores.

There are no synthetic chemical fungicides available to treat chocolate spot but plant and fish oil preparations such as Vitax 2 in 1 have some protective effects. This product will not cure an infection once it has taken hold however. There may also be some mileage in applying a spray of potassium bicarbonate to already infected plants (a food grade powder available from The Organic Gardening Catalogue) as it may halt the spread but its efficacy against chocolate spot has not been well researched as yet.