Fabaceae of large-scale crops (peas, broad bean, lupin, lucerne, clover)
Peas can be affected by several aphid-transmitted viral diseases. The pea enation mosaic virus (PEMV) can lead to 40% falls in seed production if infection occurs late and 100% if this comes early. The pea aphid (Acyrthosiphon pisum) and the strongly polyphagous green peach aphid (Myzus persicae) are the principal vectors of these through the persistent mode. They introduce the virus into the field by stimulating the formation of foci, then spread it as they move around. Between two cropping periods, the virus sustains itself in wild Fabaceae or varieties grown for fodder. Chemical control can limit dissemination of the virus from foci already established within the plot, but it does not protect against the generation of new foci by infectious aphids coming from the outside.
High-protein pea crops can be hit by another viral disease, pea seed-borne mosaic virus (PSbMV). Transmitted through the seed, this can bring yield losses in the order of 15%. Once introduced by the seed, the aphids then spread it through the non-persistent mode. Monitoring seed quality now helps to eliminate infected batches.
The other viral diseases like clover yellow vein virus (CYVV) or bean leaf roll virus (BLRV) are not so significant.
.Before flowering, the pea aphid (Acyrthosiphon pisum) can be seen on or under the leaves. It then colonizes the buds, flowers, finally the pods. As well as the damage linked to the virus transmission, the aphid harms the plant directly by extracting the sap, which leads to failure of the flower’s development and lower yield, diminished 1000-grain seed weight and fewer pods. Mild winters encourage early onset of the insect’s attacks in spring. Recorded losses can reach 20 q/ha. Alongside this species, the black bean aphid (Aphis fabae) also occurs, frequent on leaves and stems.
Aphid control measures can only be curative. It is advisable to intervene at the beginning of the rapid growth phase population, when the threshold of 30 aphids per stem is reached. This generally takes place during the flowering stage.
The major pest of the broad bean or faba bean is the black bean aphid (Aphis fabae). This species installs colonies like sleeves around the stems. It brings about substantial yield losses, particularly in the east of France when it establishes its population early.
On lupin, attacks by the lupin aphid (Macrosiphum albifrons) are found. Originally from the American continent, it was introduced into Europe in 1981 and first seen in France in 1988. It currently remains quite infrequent however.
Lucerne is affected most abundantly by the pea aphid (Acyrthosiphon pisum). A cut made in advance can considerably reduce the insect’s numbers. The vetch aphid (Megoura viciae), cowpea aphid (Aphis craccivora) and yellow clover aphid (Therioaphis trifolii) are also frequent. Two kinds of damage are caused by all these species: direct, when their proliferation is at its highest; indirect following the virus transmission. The perennial Fabaceae (lucerne and clover) can be a winter reservoir for viral diseases of the annual species like peas or haricot beans
Lucerne can suffer from several viral diseases. This is the case for example of alfalfa mosaic virus (AMV) which brings lower yields, possibly falling by 70%. This disease begins to show in spring in the form of an inter-vein mosaic and, depending on the strain of the virus, as a more or less pronounced dwarfism. The virus is transmitted by numerous species of aphids non-persistent mode and also by seeds the infected plants produce.
The lucerne enation mosaic virus (LEV) is a serious viral disease, seen since 1969. It is characterized by the formation of cock’s-comb tumours (enations) which can reach several mm high on the underside of primary and secondary veins of folioles (leaflets). It is reported in the South of France and around the Mediterranean Rim. Transmission into the field is effected by aphids, notably the cowpea aphid (Aphis craccivora), through the persistent mode From the crop selectors’ point of view, this disease is highly detrimental.
Clover is sensitive à to many viral diseases some of which are aphid-transmitted (alfalfa mosaic, bean yellow mosaic, cucumber mosaic). On clover, the most frequent aphid remains the pea aphid (Acyrthosiphon pisum). The yellow clover aphid (Therioaphis trifolii) brings on leaf withering. The honeydew colonies produce strongly favour the development of sooty mould. On the inflorescences, the clover aphid (Nearctaphis bakeri) can be found (but also the leafcurl plum aphid (Brachycaudus helichrysi).
Fabaceae of market-garden crops (faba bean, haricot bean, peas)
On broad bean the most harmful pest is the black bean aphid (Aphis fabae). It forms colonies of matt black aphids, concentrated sleeve-like on stems, mainly at the ends. It causes growth to stop, failure of flowers to develop and splitting of the most strongly attacked pods. A treatment application is recommended if more than 20% of the plants are attacked.
Haricot bean is a host to the black bean aphid (Aphis fabae) which develops on every organ of the plant, including the pods. During flowering it can induce coulures from flowers. The pea aphid (Acyrthosiphon pisum) builds colonies that are sometimes abundant in summer, on stems and leaves. That leads to aborted flowers and overall wilting. The two species present on pods cause decreased yields and a fall-back in sales (pods unfit for consumption). They also transmit viral diseases (BCMV and BYMV). The bean root aphid Smynthurodes betae can make the plant wither if attacks are strong. It is found in June on roots where it forms spherical masses of creamy white aphids.
On peas the aphid-transmitted viral diseases are also important. The pea leaf-roll virus (PeLRV) stops growth and causes yellowing of the plant, fruit setting and pod fattening are stopped. This is the most frequent virus in southern France. The pea enation mosaic virus (PEMV) strikes rather in the north of the country. These two viruses are transmitted through persistent mode by several species of aphids. Also found is a pea strain of the bean yellow mosaic virus (BYMV). The pea aphid Acyrthosiphon pisum colonizes the leaves and stipules. Early outbreaks, from the end of April, can lead to plant exhaustion, aborted flowers and reduction of seed numbers. However, these kinds of direct damage are still occasional. The damage threshold for this species is reached when there are about 30 aphids per stem apex, approaching flowering time.