Reproducible infection of intact roots of oil palm (Elaeis guineensis) with Ganoderma boninense, the cause of basal stem rot, showed penetration followed by rapid longitudinal progression of hyphae and colonization of the lower stem (bole) Light and transmission electron microscopy showed invasion of the root cortex, with no evidence of selective progression through the vascular system or lacunae In newly colonized tissue the fungus behaved as a hemibiotroph, with numerous, wide, intracellular hyphae occupying entire host cells that possessed intact cell walls and contained discernible cytoplasm and organelles In the bole this phase coincided with a complete depletion of previously abundant starch grains in advance of invasion Subsequently, in the roots and colonized stem base, widespread necrotrophic, enzymatic attack of all layers of the host cell walls occurred Hyphae were intra- and intercellular and intramural and associated host cell wall degradation was often at a distance from hyphae, resulting in cavities within cell walls A third developmental stage was the formation of an extensive, melanized, tough mycelium or pseudo-sclerotium which surrounded roots and comprised many very thick-walled cells encasing more typical thin-walled hyphae Macroscopic observation of and isolation from the bole of randomly felled, commercial palms provided confirmatory evidence that multiple infections originated in the roots before spreading into the base of long-established palms

https://bsppjournals.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-3059.2009.02100.x

Basal stem rot of oil palm (Elaeis guineensis); mode of root infection and lower stem invasion by Ganoderma boninense.

Ganoderma boninense causes severe losses to oil palm in South East Asia. The disease typically manifests itself as basal stem rot, but there remains controversy over the route of infection and source of inoculum. Using isolates differing in aggressiveness, infection via roots was confirmed; it was also shown that large inoculum provided as Ganoderma-infested palm- or rubber-wood blocks (12 × 6 × 6 cm) is necessary for soil infection of seedlings after 6–8 months. Smaller blocks (3 × 3 × 3 cm) produced rapid (≤ 3 months) infection of roots and lower stem when physically attached to roots. Therefore fragmentation of infested palm wood from a felled, mature plantation before subsequent replanting may provide inoculum. Failure of G. boninense to grow through non-sterile soil or organic debris from frond bases, suggests it is a poor competitor and that roots must contact inoculum directly. Severe disease occurred after 8 months on inoculated seedlings under shade, but not on seedlings exposed to sun. Soil temperatures in sunlight frequently rose above 40°C and reached 45°C, whereas in shade they never exceeded 32°C. Ganoderma boninense is probably inhibited in exposed soil since optimal growth in vitro was 25–30°C, and there was no recovery from 45°C. Soil temperature may explain why symptoms often first appear in mature plantations when canopy formation creates shade. Infection is not peculiar to senescing palms but can occur throughout the growth cycle.

Effects of inoculum potential, shading and soil temperature on root infection of oil palm seedlings by the basal stem rot pathogen Ganoderma boninense

Plantation forestry is of growing importance in many countries of southern and eastern Africa. With the increased planting, the threat of pests and diseases to these new timber resources has also grown. The significance of pests and diseases to forestry planta- tions not only applies to the countries in which they are being set up, but also to neighbouring states where plantations are already established. Disease surveys have previously been conducted in East Africa, but mostly more than 20 years ago. We have undertaken a series of surveys in Kenya, Malawi, Mozambique, Tanzania and Zambia to obtain information on the present disease situation in eastern and southern Africa. Some diseases known from other areas of the world were identified and many of these represent new reports from the countries in which they have been found. The diseases encountered included cankers caused by species of Chrysoporthe, Botryosphaeria and Coniothyrium zuluense on Eucalyptus species; leaf and shoot diseases caused by species of Mycosphaerella and Cylindrocladium on Eucalyptus spp.; root rot caused by Armillaria spp. on Eucalyptus, Pinus and Acacia spp.; die-back and canker on Pinus spp. caused by Diplodia pinea and on Acacia mearnsii Phyrophthora root rot and wilt caused by Cerato- cystis albifundus Some of these diseases can impart serious eco- nomic losses, so that forestry companies and organizations in East and southern Africa will need to re-vitalize or establish breeding and selection programmes to identify disease-tolerant planting stock. The preliminary surveys reported here provide a basis for the establishment of an effective African tree health network.

https://www.fabinet.up.ac.za/publication/pdfs/609-2005_roux_meke_kanyi_mwangi_mbaga_hunter_nakabonge_heath_wingfield_sa_j_sci.pdf

Diseases of plantation forestry trees in eastern and southern Africa

Efficacy of soil solarization, Trichoderma harzianum, and coffee pulp amendment against Armillaria sp

Armillaria is a primary root rot pathogen of tea (Camellia sinensis) in Kenya. The main species presently described in this country are A. mellea and A. heimii. A survey covering fourteen districts of Kenya was carried out and forty-seven isolates of Armillaria collected. Cultural morphology, rhizomorph characteristics, somatic incompatibility and features of basidiomata were used to characterize the isolates, together with molecular analysis based on randomly amplified polymorphic DNA (RAPD), inter-simple sequence repeat (ISSR), restriction fragment length polymorphism (RFLP) of the internal transcribed spacer (ITS) and the intergenic spacer (IGS) regions and sequence of the IGS region. It can be concluded that two Armillaria species were present and they were different from A. mellea. The first group was morphologically similar to A. heimii but this was contradicted by the molecular data, suggesting that A. heimii could be a complex of several species. The second group was different from the first and morphological and molecular data strongly suggest that it could be a new Armillaria species.

Characterization of Armillaria isolates from tea (Camellia sinensis) in Kenya

Surveys for Armillaria root disease severity were conducted over a 5-year period in small tea farms (0.5 to 1.0 ha) in the 12 tea-growing districts of Kenya. The disease occurred in all tea districts, but severity was greater in the districts east of the Rift Valley. Disease severity was associated with relative amounts of residual woody debris, especially roots, from trees and shrubs present when the land was converted to tea plantations. Excavation of tea bushes in disease centers showed that infection of tea bushes occurred primarily by mycelial growth from residual tree roots and from infected tea roots rather than from rhizomorphs. Rhizomorphs were scarce, and rarely involved in infection. They were confined mostly to the surface of the residual tree roots and were found growing freely in the soil in only one tea district. Rhizomorphs were more abundant in higher elevation plantations than in lower elevation plantations, where they occurred only on residual tree roots in the deeper, cooler, moister levels of the soil. Inoculum from residual tree debris in the soil was the most important source of infection in plantations of seed origin. Secondary spread from infected tea plants to healthy ones was limited and disease centers were small. In tea plantations derived from clonal cuttings, secondary disease spread from infected to healthy tea plants was more important resulting in large disease centers or gaps due to plant death and removal. Currently, soil sanitation by thorough removal of roots of forest trees and prompt removal of infected tea bushes is the best available management practice.

https://apsjournals.apsnet.org/doi/pdfplus/10.1094/PDIS.1997.81.2.133

S W Waudo

Papers

Distribution, Severity, and Spread of Armillaria Root Disease in Kenya Tea Plantations