VIRUS DISEASES OF CACAO IN WEST AFRICA

Increasing the number of Pseudococcus njalensis per test plant increased the infection rate with cacao virus 1A and 1M to 95 % with twenty-five insects. Young leaves with the red vein-banding symptoms were better sources of virus than mature leaves, and there was evidence of virus localization in the plant. It was estimated that Ps. njalensis nymphs took about 16 min. to penetrate plant tissues with their stylets.
Infection rates increased with time of test feeding up to 50 min. The virus was non-persistent in the feeding insect, but persisted up to 36 hr. in insects starved after the infection feed. Starvation before the infection feed increased the infection rate with infection feeds up to 10 hrs., after which the infection rate with pre-starved insects declined to the level attained by unstarved insects.     

VIRUS DISEASES OF CACAO IN WEST AFRICA VIII. THE SEARCH FOR VIRUS-RESISTANT CACAO

The methods used and the results of 10 years' search for cacao resistant to swollen-shoot disease are described. Selection among the trees surviving in farms devastated by this virus disease led to the discovery of mild virus strains which can protect trees against virulent strains.
When tested by graft inoculation with virulent virus, none of the selections showed any immunity or resistance save that conferred by previous mild-strain infection. A low degree of tolerance was found in some selections.
Local selections and a range of new introductions were tested by mealybug infection, and only types from the Upper Amazon region of Ecuador were consistently resistant to infection. This genetical resistance seems to be strongest in cacao from the Nanay peninsula, near Iquitos.     

VIRUS DISEASES OF CACAO IN WEST AFRICA IX. STRAIN VARIATION AND INTERFERENCE IN VIRUS 1A

Cacao virus iA, the most important and prevalent of the viruses that attack cacao in the Gold Coast, occurs in strains that differ widely in their virulence towards cacao. Outbreaks usually contain trees infected with different strains and individual trees are often infected simultaneously with more than one strain; this can be demonstrated by coppicing the trees, and by inoculating sets of test plants with grafts from different parts of one tree. Neither mild nor virulent strains seemed to be consistently dominant in roots or in other parts of cacao trees.
Cacao plants infected with mild strains were nearly always protected against the effects of infection by virulent strains; however, virulent strains entered hosts already infected with mild strains, but usually without causing any symptoms unless the plants were coppiced. The severe symptoms that developed on new growth from such coppiced plants were seldom repeated in later growth. Mealybugs transmitted the virulent strains from leaves with symptoms characteristic of infection by the latter, but not from leaves free from such symptoms. These results suggest that the multiplication of a virulent strain is impeded in plants infected with a mild strain.
In the field, infection with a mild strain protected mature trees against the effects of virulent strains spread by mealybugs. During 3 years in which 273 out of 387 previously uninfected trees became severely diseased, only 35 out of 416 infected with themildstrain developed symptoms of infectionwiththevirulent strain. Five years after infection with the mild strain, trees were yielding I pod per tree more than in the year they were infected, whereas the decrease on trees infected with the virulent strain was 16 pods per tree. Some limitations in the practical application of protection by mild strains, and objections to its use as a control measure, are discussed.     

VIRUS DISEASES OF CACAO IN WEST AFRICA: I. CACAO VIRUSES 1A, 1B, 1C AND 1D

As an introduction to this series of papers, a brief history is given of swollen-shoot disease of cacao in the Gold Coast, and the nomenclature for cacao viruses is discussed.
The symptoms of four virus diseases of cacao are described in detail, together with experimental data under the headings: cross-immunity reactions, latent period, seed transmission, staining reaction, heat inactivation and rate of spread.     

VIRUS DISEASES OF CACAO IN WEST AFRICAL: II. CROSS-IMMUNITY EXPERIMENTS WITH VIRUSES 1A, 1B AND 1C

Experiments on cross-immunity reactions between three viruses attacking Theobroma cacao L. on the Gold Coast are described. A field trial involving 3 acres of graft-inoculated trees revealed some degree of protection afforded by Theobroma virus 1B against infection with virus 1A. The protection appeared to be more effective against insect inoculation than against graft transmission, being only temporary in the latter. Virus 1C (probably unrelated but not to be called Theobroma virus 2 until more evidence is available) conferred no protection against virus 1A.
The latent periods for these viruses were calculated from this experiment, which also provided data on their effects on yield. Virus 1A reduced yield by 50% in the first year after inoculation and killed the trees in the second. Virus 1B had no appreciable effect on yield; virus 1C reduced yield by 50 % in the third year after inoculation but there was no further decline in the fourth.
The rates of spread of these viruses were compared and significant differences demonstrated.     

VIRUS DISEASES OF ADANSONIA DIGITATA L. (BOMBACACEAE) AND THEIR RELATION TO CACAO IN GHANA

Virus-infected Adansonia digitata L. were found in the Guinea-savannah woodland forming the Northern Territories and on the Accra plains of Ghana, but those tested in the rain forest were not infected. Three viruses isolated from infected trees resembled the Kpeve cacao virus, but no definite relationship could be established. It is unlikely that the original outbreaks of swollen shoot in cacao came from A. digitata.     

NOTES ON A VIRUS DISEASE OF CACAO

A virus disease of cacao, discovered in Trinidad towards the end of 1943, is apparently confined to the north-western corner of the is land and does not occur in the main cacaogrowing areas of eastern, central and southern Trinidad; nor has it been found in Venezuela, Tobago or the Windward Islands of Grenada, St Vincent, St Lucia and Dominica. The symptoms resemble those of West African swollen shoot disease, consisting of vein-clearing or mosaic of the leaves and red-mottle of the leaves and pods. No swellings are formed, but recent observations indicate that the disease may cause both dieback and reduction in yield. There are two strains closely resembling each other. The spread of the disease under field conditions has been demonstrated, and although the rate of spread has varied somewhat, a mean increase of 41 % over the original number of infected trees has been found over a 10-month period. As with swollen shoot disease, existing diseased areas have increased in size and new outbreaks at short distances have occurred. Both strains were readily transmitted by budding, the incubation period varying from 34 to 136 days, with a mean around 90 days, but no insect vectors have been found. There is at present little reason to believe that any varieties of cacao are immune to the disease or are even symptomless carriers. The question of eradicating this virus, while its distribution remains limited, should be seriously considered.     

THE PHYSIOLOGY OF VIRUS DISEASES IN PLANTS

In this paper the movement in the plant of the causative agent of virus disease is discussed. The relevant data in the literature are summarised.
A method is described whereby a portion of the stem in the middle of a tomato plant was killed either by chloroform or by steam. In this way the living upper and lower portions of the plant were connected by a bridge of dead tissue. It is shown that the symptoms appeared in that part of the plant in which the inoculation was made. The virus agent did not travel across the dead region.
The xylem tracts were not materially affected by this treatment, and water travelled across the region. Evidence of this is the fact that the distal portion remained turgid and sometimes continued growth for a considerable time. If the stem were removed above the ground level and put into eosin solution, this travelled readily over the dead tissue. That the vessels were not occluded by protein plugs is shown by the fact that particulate substances were carried up the xylem tracts past the dead region.
No evidence of adsorption of the virus agent to the cell remains could be adduced, so it is assumed that it was not travelling in the xylem stream.     

STUDIES IN RUBUS VIRUS DISEASES

Raspberry leaf mottle virus is acquired by Amphorophora rubi in feeding periods of 1/2-2 hr. Longer feeding periods cause no significant fluctuations in infectivity. During continuous feeding on healthy plants, infectivity declines rapidly after 2 hr., but persists for at least 5 hr. if aphids are transferred frequently to fresh healthy plants. The estimated probability of infection by single aphids is 1 in 20. Rarely more than 50% of test plants, Rubus occidentalis , seedlings became infected, and this is thought to be due mainly to variation in plant susceptibility. Young leaves of infected Norfolk Giant are, however, better sources of virus than older leaves.     

STUDIES IN RUBUS VIRUS DISEASES

A chlorotic veinbanding disease of raspberries is shown to be due to a virus transmissible by the aphid Amphorophora rubi Kalt. after infection feeding periods of 18 hr. or more.
This virus causes leaf symptoms, masked in hot weather, on a wide range of European and North American varieties. Symptoms on the latter are analogous to those of the red raspberry mosaic of American authors.
The names veinbanding disease and raspberry veinbanding virus are proposed.     

STUDIES IN RUBUS VIRUS DISEASES

Yellows diseases of raspberry, previously considered to be physiological disorders, have causes, presumably viruses, that are transmitted by grafting. Several can be distinguished by the severity of symptoms produced in different raspberry varieties, but whether they are caused by distinct viruses or by related strains is uncertain. Two, called mild and severe yellows, are thought not to be related to the yellow mosaic and yellows virus described in North America.     

THE PATHOLOGY OF SOME DISEASES FOUND IN WILD ANIMALS IN EAST AFRICA

Fifty-four post-mortem examinations were carried out on wild animals. A wide range of disease conditions was found and a number of these are discussed in detail. Two conditions of particular importance were parasitic bronchitis and muscular dystrophy; these were found in approximately one third of the antelopes examined.