Sweet pea leaves have a thick waxy coating which gives considerable protection against pathogens. Consequently, sweet peas are less susceptible to pests and diseases than many garden plants. They are most vulnerable as seedlings, before the waxy surface has had time to develop, or when the plants are old or under stress when the natural immune system is breaking down.
Sweet Pea Diseases
A seed borne fungal disease reported from the southern USA, but not recorded in the UK. Small white spots on the leaves, flowers and shoots which spread rapidly, causing affected parts to wilt and die. Attacks the younger growth initially, and then spreads downwards. Young shoots become pale and brittle.
A common leaf spot which can be difficult to control. Damp dull conditions encourage the disease to spread. Systemic MBC fungicides with added wetting agent may give some control. This disease is seed borne, so seed should not be collected from infected plants.
Black Root Rot
A very widespread soil borne fungus attacking a wide range of plants. Symptoms vary, but in sweet peas it shows as a black discolouration of the roots. Although development of the disease is most rapid in the temperature range 20°c - 25°c it seems to establish itself when a plant is under stress and is favoured by cold wet soils and poor hygiene. Excessively high air temperatures can also render a plant liable to infection. Discouraged by acid soils - it cannot survive in soils with a pH below 5.6. Strongly growing plants can carry a moderate level of infection without showing symptoms.
Botrytis (Grey Mould)
Most obvious in dank weather when it causes flower spotting. Mid and deep blue varieties are particularly susceptible being disfigured by white spots. Certain white varieties readily show brown spots, but others are more resistant. This very common and widespread disease is often a secondary infection on Fusarium or on physical damage and then appears in the form of a grey fluffy mould which can spread to kill the plant. Difficult to control chemically, but fortunately the most aggressive strains are not the most resistant to treatment. Generally speaking, an improvement in ambient conditions will produce the most favourable results. The disease likes mild damp conditions with stagnant air, and consequently is discouraged by air movement, a buoyant atmosphere, and high or low temperatures.
Less common than powdery mildew, this can be serious and difficult to diagnose in its early stages. Seedlings showing weak thin growth with small leaves tending to curl upwards and inwards and showing a 'blistered' surface on plants with good roots and a clean neck should be suspected of carrying this infection. A faint greyish deposit on the underside of the leaves offers confirmation as the disease progresses. Subsequently irregular pale patches develop on the upper leaf surface, with a pale brownish or greyish 'felt' on the corresponding areas of the underside of the leaf. Favoured by cold damp conditions and poor air circulation.
This appears as irregular powdery white spots on the upper surface of the lower leaves. Can be destructive in dank weather and on older plants. Best controlled by fungicides containing Pyrifenox. Cutting off old lower leaves of sweet peas may help by improving air circulation, and reducing the source of further infection.
Ramularia Leaf Spot
This disease is specific to sweet peas. Large tan spots without definite margins appearing initially on the lower leaves. Infected leaves often fall. The disease survives in sweet pea refuse and is favoured by wet conditions. Uncommon.
Known as 'Cottony Rot' in the USA. Several related disease organisms, more often affecting field crops, notably oilseed rape. Spores can lodge in the junction of the two leaflets on a sweet pea leaf, usually in late spring, causing an innocuous looking rot which will rapidly spread back up the petiole and into the main stem, killing the plant. Cutting off the infected leaf solves the problem if carried out promptly before the disease starts to spread.
Reddish streaks appearing on the haulm. Tomato Spotted Wilt Virus can produce similar symptoms. Formerly common on sweet peas but becoming rare.