A RECENT OUTBREAK OF SECONDARY LEAFFALL DUE TO OIDIUM HEVEAE.
BY
F. BEELEY.
During- the re-foliation period, after the heavy wintering- of rubber
trees this year in Malaya a larg-e number of enquiries was received,
at the Institute from estates in connection with a secondary leaf-fall.
All specimens of leaves and flowers received for examination
showed that the material was infected with Mildew disease caused
by the fung-us Oidium Heveae.
On account of the severe character of the outbreak of the disease
in two districts, I was requested by the Director to visit estates and
small holdings in these districts and to report on the situation.
The following is a report of the results of the observations made
during the tour.
While travelling from one place to another a keen watch was
kept for any signs of the disease in small holdings and kampongs
by the roadside. As a result of this it can be stated that a heavy
infection of Oidium Heveae in all areas lying on the route taken was.
observed. In many cases the presence of the fungus was demonstrated microscopically. This was best done on leaves an'd flowers
picked fresh from the tree.
Following recent rains considerable
difficulty was experienced in demonstrating the active sporulating
fungus on the leaves, but on no occasion was difficulty encountered
in demonstrating A vigorous sporulating growth of the fungus on
the flowers and flower stalks. It is presumed that the natural hirsute
condition of the flowers affords a means of protection from the elements, and incidentally a means of carrying over infection from one
set of new leaves to the next despite occasional heavy showers of
rain in the meantime.
The following facts were noted:—
1.
Verv few inflorescences remained on the tree.
2. Very few flowers were observed to open to maturity.
3. Early wintering trees had, almost without exception, failed"
to set fruit.
4. Late wintering trees though bearing a heavy flush of flowerswere all so heavily infected with the fungus that few if any flowers
would reach maturity.
5. The mildew fungus usually attacks only young leaves of say
from J to 2 inches long. Investigators in Ceylon and the NetherlandsEast Indies, however, report that older leaves may also be attacked01
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6. Leaves may apparently recover from an attack, but remain
for a long time in a weak pendant condition, eventually extending
to give malformed, wrinkled, pale or mottled leaves, many of which
are so weak as to be forced off by subsequent wind or rain.
7. It has been frequently observed that in advanced cases, when
the leaf is beginning to discolour, great difficulty may be encountered
in demonstrating the presence of the Oidium fungus, while other
secondary fungi may often be found on such specimens. It seems
reasonable to conclude that the fungus ceases its activity on the leaves
as soon as the leaf begins to die, or as soon as the leaf has become
old enough to have acquired a cuticle which, so far as this fungus
is concerned, may be regarded as impenetrable.
8. Advanced cases in which the disease resembles "white wash"
spots on the leaves as are sometimes found in the Netherlands East
Indies and Ceylon were not found on the trees in the districts visited.
Similar specimens found on the ground when examined microscopically
showed the white wash effect to be due to other fungi commonly
found on decaying vegetable matter.
The
writer
has
successfully
•photographed an extraordinary dense growth of the Oidium fungus
•on the leaf vein, where the conidiophores and conidia were so dense
as to be in contact with each other and bearing one to three spores
in chainlike formation, and yet the growth was visible to the naked
•eye only when observed in certain angles of light, when a slight
glistening furry surface could be distinguished from an otherwise
smooth leaf surface. On the flower stalks a glistening powdery appearance characterises the presence of the mildew. (See Plate,
page 65).
9. The extent of actual leaf fall seemed to be in proportion to
the density of foliage normally produced by the trees. Trees in good
soil bearing a heavy foliage suffered a heavier fall of leaf than trees
;
in poorer soil bearing a lighter foliage. A far more rapid recovery
had however taken place in the heavier foliaged areas. The flowers
in all cases were almost completely destroyed.
10.
The mildew was demonstrated microscopically on leaves of
trees of all ages over z\ years.
Several plants of younger age in
new clearings were observed to be suffering from attack by mites and
leaf spot fungi, but this may be due to the drought and not to an
.after-effect
of an attack by Oidium Heveae.
Partial
wintering'
of
young trees of 2—4 years old made the attack in such clearings
appear only in a few branches the leaves on which were sufficiently
"immature to be susceptible.
11. Cultivation or schemes for soil conservation had, where
.adopted, produced a more rapid recovery from the disease. This can
'be shown by the quicker rate at which leaves in such areas had assumed the semi-rigid extended position, while on uncultivated areas
and on poor soils the leaflets, though of normal size for their age,
remain hanging vertically.
63
12. A few trees were observed to have their second flush of
young- leaves heavily infected with the fungus. Such foliage was
light, weak and pale yellow green in colour.
13.
No special type of tree appeared to be affected to a greater
extent than any other type.
The taller trees appeared to suffer more
than those of normal height within the same area. Closely planted
small holdings' rubber appeared to have shed more leaves than more
widely planted estate rubber, as evidenced by the amount of green
leaf on the ground. This quantity of green leaf on the ground may
however be due to the more humid atmosphere within the more densely
planted areas and hence a slower death and decomposition of the
young fallen leaves.
14. Soil types, cover crops and period of resting from tapping
did not appear to have any influence on the incidence or severity of
the disease.
15. Manuring also did not seem at first sight to have any great
effect on the severity of attack. Dr. Haines, Head of the Soils
Division, informed me of the various estates in the districts visited
which were carrying out manurial experiments, and special attention
was paid to these. In a few cases wintering had been delayed sufficiently by the nitrogenous manure applied to make the arrival of theyoung leaves coincide with the arrival of wet weather, and consequently these had escaped the disease to some extent. The flowers in such
areas were almost completely destroyed despite the rains. Other
manured areas owing to their geographical position had not had rain
so early, and had suffered as badly as other non-manured areas. It
would seem therefore that an increase in vigour of a tree by natural
or artificial means does not render the small leaves immune to attack
by the mildew fungus.
16. No immune trees were observed, always one could observe
some "branch of the tree which, wintering later than the others, could,
still be seen to be suffering from the disease.
17. The normal wintering period for the district is one month
from the first week of February to the first week in March. Owing
to the recent mildew attack many trees in the district are only just
getting over the wintering period in April, due to the destruction of
the earlier leaves formed previous to the advent of wet weather in
later March. Since the rain however, the trees are assuming a foliagewhich is perhaps less dense than in normal years, and which will in all
probability fall off at an early date and be replaced by other foliage
about the months of July—September as is often experienced in dry
districts.
18. Areas of known infection consist of practically the whole of
Selangor, Negri Sembilan, Malacca and North Johore.
Flower infection is complete in all districts, but the leaf infection
is most serious in Malacca and southern N,egri Sembilan and decreases
64
with the increase in distance from these areas.
Though leaf infection
has been demonstrated many times in Selangor, very little if any leaf
fall has actually occurred; its effect, however, can be seen, as in the
unfallen leaves of trees in Malacca, in the form of malformed, twisted,
mottled, pale green leaves.
19. The association of mites with Oidium Leaf Fall was not
borne out by my investigation. Mites were present only in advanced
stages of the disease, when the mildew fungus was difficult to find
and when decomposition had set in. Apparently healthy young greenleaves and flowers were observed to be heavily infected with the
fungus, while mites were not present.
20. Owing to the very widespread infection, the method of
spread could not be determined. In all probability infection is caused'
by wind borne spores. No clue as to the type and location of a rest-
ing spore was found.
Experiments are however being carried out hi'
the laboratory with a view to determining the method of formation,
type and viability of the resting spore of the causal fungus. The difficulty of the investigation is increased by the fact that as yet no meanshave been discovered for the artificial culture of the fungus.
21. The amount of rainfall seems to have an all-important influence on the severity of attack, especially on the leaves. The following table will show the differences in rainfall for the first quarter of"
the years 1929 and 1930. In 1929 very few cases of mildew were
recorded. In 1928 a low rainfall and correspondingly numerous cases
of mildew were recorded. In the table below, the Asahan district is
recorded as having a greater fall of leaf than neighbouring districts,
despite its higher rainfall. This is probably due to poor rubber and
poor soils and hence a slower and poorer recovery from winteringand disease.
District
Rainfall
Eainfall
Leaf
1929
1930
Flower
Jan-March. Jan-March. infection Infection
(inches)
(inches)
Extent of
Leaf fall
K. Lumpur
18.5
20.0
Very
heavy
Slight
Very slight
N. Sembilan
21.25
6.1
do
Heavy
Moderate
Tampin
23.5
6.2
do
do
Jasin
15.84
8.84
do
do
do
Asahan
16.77
9.02
do
do
Considerable
Tangkab
2] .01
8.07
do
do
Moderate.
•
do
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22. The possible effect of the disease upon the yield of rubber
per acre was an important part of the investigation. The results of
this part of the work are, however, inconclusive.
The removal of restriction in November, 1928, change of system
of tapping, change over from lower to high panel, introduction of
soil conservation schemes, lowering of number of trees per tapping
task, change over from Chinese to Tamil tappers or vice-versa, the
unusual drought and introduction of periodic resting of trees from
tapping^ are all factors which affect the yield per acre and were
encountered to a greater or less extent on the estates visited. It
was therefore deemed advisable not to attach any importance to the
figures obtained many of which are extremely contradictory.
It
seems reasonable to suppose however that the Mildew Disease of
rubber trees will prolong the lowering of yield effect of normal wintering for at least one month. This fact is borne out on estates where
soil conservation work had not been carried out in recent years, by
.a comparison of yields for 1930 with those of similar months in previous years. Here again the abnormal drought may be the chief
factor concerned. If an appreciable loss in yield of say 25 Ibs. or over
per acre can be shown as a direct result of the mildew attack, then
in subsequent years, when the two months previous to wintering are
abnormally dry, treatment by means of sulphur dust should be of
•commercial value.
Meantime it must be realised that experience so far shows that
•climatic conditions in Malaya are unsifted to a prolonged activity of
the fungus and therefore extensive losses due to a mildew attack are
not to be expected.
The rains recently experienced in the districts visited had almost
•completely checked the activities of the fungus and rapid recovery of
diseased leaves was taking place.
The chief effect of mildew attack will be felt in the seed harvest.
In the brief time at my disposal I was unable to record having seen
a single young seed, while extremely few flowers were observed to
have developed beyond the bud stage. All floral specimens examined
showed a heavy infection of the fungus.
Lack of seed for future
planting programmes is therefore more likely to cause anxiety than a
possible appreciable reduction in yield.
DESCRIPTION OF PLATE.
The fungus Oidium H'eveae photographed in its natural state on
the midrib of a very young leaf of Hevea Brasiliensis. x 114.
Note,
(i)
Chains of spores at the end of the upright, unbranched
conidiophores.
(2)
Spores germinating in situ on the conidiophore.
(3)
Density of growth which to the naked eye appeared
merely as a silken sheen on the midrib regions of the
underside of the leaf.
Photo by H. Sittcliffe
and F. Beeley.
Oidi-um Heveae.