Many factors can affect growth of the pea crop, and the notes below describe the main pests and diseases which reduce yield and quality. Further information can be found in PGRO Technical Update TU15 ‘Checklist of Fungicides and Insecticides for Combining Peas’.
Weevil may cause damage if large numbers appear when plants are small and in particular in cloddy seedbeds and in conditions of slow growth. Leaves of attacked plants show characteristic ‘U’ shaped notches around the edges, but the main damage occurs as a result of the larvae feeding on the root nodules. Sprays may be applied at the first sign of leaf damage and repeated after 7 - 10 days.
A monitoring system is available from Agralan Ltd (The Old Brickyard, Ashton Keynes, Swindon SN6 6QR) to predict the likely severity of attack.
Field thrips feed on the leaf surface of emerging seedlings which results in a thickening and puckering of the tissue. Seedlings may appear pale in colour. Although further damage can be checked by spraying, in the majority of cases the peas will outgrow the effects of thrips, and yield improvement may not be achieved following treatment.
This disease produces resting spores, which persist in the soil and initiate primary infections in young pea plants. Though secondary infections can develop, particularly in cool, damp conditions, they are rarely as damaging as primary infections, which can kill plants before flowering. Fungicide seed treatment should be combined with varietal resistance to avoid serious losses. There are no foliar fungicides which give effective control.
Aphids can cause severe yield loss when present in large numbers, and early infestations can result in crops becoming infected with pea enation mosaic virus. Aphids should be controlled as soon as colonies can be found on 20% of plants, particularly where crops have commenced flowering. Yield can be improved by controlling aphids at any stage up to the time when four trusses of pods have been set.
Pea cyst nematode is a very persistent soil-borne pest, often causing severe yield loss. Frequent cropping of peas and Vicia faba beans favours the build-up of infestations, and an adequate rotation is essential to minimise the risk of occurrence. Affected plants are stunted and pale, and the root systems do not develop nitrogen-fixing nodules, but become studded with white, lemon-shaped cysts. Correct diagnosis is essential as subsequent pea crops grown in infested fields are subject to complete failure.
Marsh spot is a disorder of peas, which is due to deficiency or unavailability of manganese. The deficiency causes the formation of a brown spot in the centre of many of the peas produced, and the produce is spoilt for human consumption and for use as seed. It is particularly associated with organic and alkaline soils. When symptoms appear in a crop, 5 kg/ha of manganese sulphate with a wetter, or an equivalent application of a manganese spray, should be applied at once in a high volume of water. Similar treatment must also be carried out when an affected crop is at first pod stage, and repeated 10 - 14 days later, in order to prevent the formation of marsh spot. In some seasons flowering is prolonged and a third manganese application will be necessary. The amount of manganese in some formulations (e.g. chelated manganese) may be too low to be effective at the rate recommended.
These feed upon the developing seeds within the pod. Yield loss is minimal, but the effect on quality can be dramatic. Damage to the seed reduces the value of the produce. A pheromone pea moth trapping system should be used to assess the need for treatment and to forecast the date on which insecticides should be applied. To further assist growers in the use of their own traps, information is provided on optimum spray dates. Growers achieving a threshold catch in their traps can obtain a predicted date for spraying in the area by going to the PGRO web site (www.pgro.org). Pea moth traps are available from several suppliers - please see PGRO Technical Update Number 3 for further details.
Leaf and pod spots are caused by three fungi, Ascochyta pisi, Mycosphaerella pinodes and Didymella pinodella, which may be spread by seed infection, soil or plant debris. The most frequent is M. pinodes, which can cause losses in both yield and quality in wet conditions. The use of disease-free seed will help to reduce the incidence of disease. There are no minimum standards specified by the statutory seed certification scheme for M. pinodes but seed, especially farm-saved, should be tested. Seed treatments are recommended for the control of disease at certain levels of infection. Fungicides such as azoxystrobin, metconazole, boscalid + pyraclostrobin and cyproconazole + chlorothalonil give useful control of the disease in the crop and can give yield increases when applied during flowering and pod set.
This can affect stems and pods during wet weather, and is initiated when petals adhere to plant parts after pod set. One or two applications of fungicides at pod set and at the flat pod stage may be required to prevent Botrytis infection when wet or damp weather occurs during flowering. It may be necessary to select products which combine control of Botrytis and Mycosphaerella. However, in dry conditions, sprays during flowering are unnecessary. Suitable products include cyproconazole + chlorothalonil, boscalid + pyraclostrobin, cyprodinil + fludioxonil and azoxystrobin.
Occasionally late maturing crops may become covered with a grey-white film of powdery mildew. The disease can delay maturity. Cyproconazole + chlorothalonil applied for leaf and pod spot control will reduce powdery mildew. Sulphur formulations with Extensions of Authorisation for Minor Use (EAMU) for peas can be used to control powdery mildew.
A soil-borne disease which can occur in any pea growing area, but is generally confined to fields with a very long history of peas. It can cause substantial reductions in yield, but is effectively controlled by genetic resistance. Race 1 appears to be the most common form. The majority of varieties are resistant to this race and growers using land in known high risk areas should select these.
Several species of fungi cause foot and root rots. The effects of these diseases are particularly common on heavy land with a history of frequent pea cropping. Good drainage and avoidance of compaction can help to minimise losses. A soil test, which predicts the likelihood of soil-borne disease causing serious yield loss in future crops, is available. There are no means of controlling foot rots satisfactorily once they become established in a field, other than extended cropping with species other than legumes. Sclerotinia sclerotiorum causes a stem rot rather than a foot rot, but affects peas, spring beans, oilseed rape, linseed, and sometimes potatoes and certain field vegetables. This should be remembered when planning rotations in areas where Sclerotinia has occurred. Cyprodinil + fludioxonil or azoxystrobin applied at first pod can give useful control.
This is a potentially serious seed-borne disease, which can occur on all types of peas. Symptoms consist of water-soaked brown lesions on the lower leaves, stems and stipules, and become noticeable following periods of heavy rain, hail or frost. The lesions may coalesce and show a fan shape on the leaf, following between the lines of the veins. Some pod spotting may occur. Severe infections have not occurred in spring-sown peas and effect on yield has been negligible.