Colletotrichum gleosporioides (Penz.) Penz. and Sacc, the perfect stage being Glomerella cingulata (Stonem.) Spauld. and Schrenk.

Distribution

IRRDB surveys show that this is the most widespread of all the diseases. There are reports of serious occurences from all three Continents where rubber is cultivated. The most recent survey lists it as severe in Brazil, Cameroon, China, Gabon, Guyane, India, Indonesia, Malaysia, and Vietnam. In an earlier Survey, one part of Brazil and most of Gabon the disease is respectively regarded as "very severe" and "devastating".

In the remainder of Brazil and all of China, Indonesia and Malaysia it was regarded as an endemic and severe problem whilst in Côte d'Ivoire, India, Sri Lanka and Thailand occasional sporadic but serious outbreaks occur. Only Philippines regarded the disease as unimportant even though its presence in this area as well is acknowledged [the Philippines did not particpate in the later Survey]. All ages of tree, from nursery to very mature, can be affected. It follows that regular and thorough control measures are essential virtually everywhere. For results of 1998 Survey.

Symptoms

C. gleosporioides attacks the leaves mainly during budburst or shortly after. Initially, blackish-brown spots emerge in the centre of the young leaves, this is followed by the affected leaves wrinkling up before suffering wet rot and finally leaf fall. At this stage, the disease is often confused with Powdery mildew and Bird's eye spot. Older leaves usually have a degree of resistance and are affected mainly around the tip and rim or are severely spotted. When the diseased part shrivels and drops away, the remainder of the leaf can be retained. Spots on surviving leaves are circular, 1-2mm diameter, with a haloed thin brown margin. These spots change on ageing and stand out like small cones to produce an apparently embossed surface. Confirmation of the disease is demonstrated in wet weather by the pale orange/pink cheesy spore masses which are produced on surface necrotic lesions.

Severe attacks in nurseries can result in leafless trees, growth retardation and budding difficulties due to bark changes. On mature trees, production levels are affected and trees suffer die-back.

Spread

The fungus, C. gleosporioides, is an important plurivorous, saprophytic pathogen which affects many of the standard wild and cultivated plants which grow in the tropics and is common on decaying plant tissue. The pathogen has a latent period and spores can survive for long periods of time. The Hevea strain can attack cocoa but not vice versa. It belongs to the Deuteromycotina Division, Melanconiales Order and generally causes anthracnose symptoms on plants.

Spores are elliptical, monocellular and transparent and great numbers are produced in aservulus. Spores are spread by rain splash and in currents of damp air, colonies are orange in colour. The mycelium can live in any dark or light environment which can provide carbon, nitrogen and vitamins. Growth occurs at any pH in the range 4-10, though 6.5-7 is the optimum, and at any temperature in the range 0-35°C, the optimum being 26-29°C. Optimum germination is at 28°C.

A relative humidity over 96% is essential for optimum "normal" spore generation but below 96% humidity germination is very low and the form of the sprout is "abnormal". Infection occurs when humidity exceeds 96%, ambient temperature is in the range 26-31°C and there is a film of water on the surface of the medium. Incubation takes 4 hrs at normal humidity and, after this germination, tissue penetration occurs 8 hrs later. Appresorium forms on the medium surface before penetration.

The pathogen penetrates directly through the cuticle, destroying the cell wall in the process, placing itself inter and intracellular. The chloroplast is the first organ to be damaged, followed by the mytokindria and then the cell itself. The pathogen releases polygalacturonase enzyme, cellulose, pectin methylesterase and toxin during the infection process. Symptoms become visible between two and four days after inoculation.

Control

Successful control requires an integrated use of cultivation technique, clonal selection and chemical control.

For transmission to occur the pathogen requires that the environment is favourable, that a susceptible plant is available and that the plant is at a critical stage, namely that bud and young leaf formation is taking place.

Environment

A favourable environment is an ambient temperature beween 26-31oC, a humidity above 96% and lengthy periods of breezy, rainy days. Obviously, little or no control is possible over such weather parameters but after periods which match these conditions, it is essential that close monitoring is carried out and control measures put in hand. Conversely, the ultraviolet component of sunlight has a very significant impact on pathogen growth and hence vigilance can be relaxed after periods of hot, sunny days.

Cultivation techniques can assist in reducing the impact of the disease even in areas where the weather pattern is unhelpful. Intensive weed control and improvements to drainage are useful ways of restricting the collection of moisture and tranferring it away to where it can do no harm and thus reducing humidity locally. Cultivation using 50% extra fertilizer not only accelerates the rate of bud and leaf formation and thus reduces the period when the plant is at its most susceptible but also promotes healthy plants.

Clones

Clonal selection can improve matters by the avoidance of those clones known to be susceptible and the planting of those known to be resistant particularly in areas where unfavourable weather patterns occur.

Susceptible clones are said to be: GT1, PR 300, PR 303, PB 5/63, LCB 1320, PB 86, RRIM 526

Resistant clones are reputed to be: AVROS 2037, RRIC100, RRIC 102, RRIC 110, RRIM 600, TM 2, TM 4, TM 6, TM 8.

Chemical control

Chemical control with fungicides is aimed at depressing pathogen population thereby reducing the rate of infection.

Copper based fungicides are most favoured though they are not suitable for mature trees since copper can pollute the latex. Application can be by spraying or dusting in the nurseries or field or by dusting or fogging on mature and immature plants. The latter is recommended with large areas of mature and immature plants.

Suitable copper-based fungicides, with recommended dosages in kg/ha are Maneb (1.2), Zineb (1.2), Maneb+Zineb+Ferbam (13:2:2,1.5), Carbendazim + Mancozeb (31:369,1.0) Mancozeb (1.2), and Chlorotalonil (1.5).