Two strains of a virus disease, “A” and “B,” which possibly may be two distinct viruses, occur on cacao in Trinidad. These are compared with the more virulent “swollen-shoot” and related viruses that are widespread in West Africa.
The most important resemblance between the Trinidad and the West African viruses is that both are carried exclusively by mealybugs of the family Pseudococcidae. There are also points of similarity in the symptoms, which in the Trinidad virus consist mainly of a transient red vein-banding, with or without a more or less discontinuous yellow vein-flecking (which does not disappear when the leaf matures) and, on certain varieties of cacao, red-mottle on the pods. Swellings on the shoots, a conspicuous symptom of most of the strains of the West African viruses, have not been observed in Trinidad.
Four species of mealybugs are definitely known to be vectors: Pseudococcus citri, which is the commonest and undoubtedly responsible for most of the natural spread of the disease; P. brevipes; a species near P. brevipes but almost certainly distinct; and Ferrisia virgata. Certain other mealybugs have been found on cacao in Trinidad but the virus has not yet been transmitted by them.
Most transmission experiments have been made with mealybugs bred up on potato tubers and with Posnette's technique of feeding potential vectors, after infection-feeding, on cacao beans from which one of the cotyledons has been removed, so that the insects can feed on the convoluted surface of the remaining cotyledon or on the radicle. Notwithstanding some drawbacks to this method it has many advantages over using young growing plants as tests.
The “latent period” of the virus in the test plants grown from dissected beans has varied between 20 and 123 days, though the symptoms most commonly appear from 30 to 50 days after infection of the bean. The symptom of red vein-banding has appeared much more often than that of yellow vein-flecking, irrespective of the symptoms on the virus source plant. When both symptoms appear the red veinbanding almost always shows up first.
All three immature instars, and young adults, of all four vector species are probably almost equally efficient as vectors, except that there is some indication that P. brevipes may be slightly more efficient than the other three species.
Starvation of the mealybugs before infection-feeding does not increase their capacity to transmit, though it usually helps to make them settle and feed more readily on the source plant.
The time for which the mealybugs feed on the virus source plant has little if any effect on their capacity to transmit the virus. Mealybugs have become infective in just over one half-hour infection-feeding, though the proportion of transmissions obtained when the infection-feeding time has been between one-half and two hours is rather less than that for over two hours, probably because in the former tests some of the insects had not actually fed.
The duration of feeding on the test bean is probably also immaterial, provided of course that the mealybugs do actually feed. The shortest test-feeding time that has resulted in transmission is 90 minutes, but this included a considerable “settling” time.
Transmission may be effected whether a mealybug feeds on the cotyledon or the radicle of a test bean.
Mealybugs can still transmit the virus if they are starved after infection-feeding for a period up to 22½ hours, but no transmission has yet been obtained when the post-infection starvation has been over 23 hours. It is not yet known whether a short period of feeding after infection-feeding renders the mealybugs incapable of transmitting, but post-infection feeding for 66 hours does so.
A transmission rate of just under 14 per cent, has been obtained with single mealybugs to each test bean but with larger numbers the rate has not risen in accordance with the mathematical expectation. A possible explanation for this might be that there are “active” and “inactive” races of P. citri, but so far practically no evidence for this has been obtained.
Mealybugs can become infective by feeding on the symptom-free parts of flush leaves showing symptoms in other parts, on entirely symptom-free flush leaves from infected trees, on the stem of a young infected plant, and on the leaves of a young infected plant after the disappearance of the transient symptoms of red vein-banding. It appears, however, that they pick up the virus more readily from flush leaves actually showing symptoms, and these have been used as the source of the virus in most of the experiments.
Few experiments have yet been made with strain “B” of the virus, but this strain has been transmitted with P. citri and P. brevipes.
All stages of P. citri, but especially young adults, often wander about of their own accord and are thus capable of spreading the virus, particularly to trees actually in contact with an already infected tree. The transport within a plantation of cacao pods during harvesting is thought to be a likely cause of isolated new infections.