In this section:
Nematodes and Soil-Borne Fungi
G.E. Walker, SARDI Plant Research Centre, GPO BOX 397 Adelaide SA 5001
Few problems caused by nematodes and soil-borne fungi have been reported in olives in Australia, and olive trees are regarded as generally hardy and quite tolerant of many soil pathogens. However, experience with olive pests and diseases is limited due to the comparative infancy of commercial production in Australia. The opportunity exists for careful selection of land for new olive orchards to avoid ground infested with high levels of potentially damaging pests and diseases; developers should find out as much as possible about cropping history as this is an important guide to what soil pathogens may be present. Land with a long history of vegetable production, especially of the tomato/ potato family, poses a particular risk as these crops are often associated with both nematodes and the soil disease Verticillium Wilt. Soil (and, if possible root samples) can be submitted to diagnostic services for nematode testing before planting olives. Although pre-planting soil fumigation can be used to disinfest soil, it is preferable to avoid land infested with pathogens known to attack olives as fumigation is expensive, rarely if ever 100% effective, and some highly effective fumigants such as methyl bromide are being gradually withdrawn because of environmental damage. Careful selection of land and clean planting material can avoid many costly problems. Nursery accreditation/ hygiene schemes provide growers with the assurance that they are buying quality planting material free of pests and diseases. Root rot fungi such as Phytophthora spp. can be readily spread in planting material.
Plant parasitic nematodes recorded on olives in Australia (Cook and Dube, 1989; McLeod et al., 1994) are several species of Root knot Nematodes (Meloidogyne spp.), Citrus Nematode (Tylenchulus semipenetrans), Root lesion Nematodes (Pratylenchus spp.) and also 2 nematodes of probable minor importance, a Sheath Nematode (Hemicycliophora arenaria) and a Needle Nematode (Paralongidorus eucalypti). Soil-borne fungal pathogens recorded on olives in Australia (Morschel, 1961) are Verticillium dahliae (cause of Verticillium Wilt) and Phytophthora cinnamomi (cause of root rot). Some olive diseases have not been recorded in Australia: keep Australia free of these diseases, do not make illegal imports of planting stocks.
Root knot nematodes
These nematodes are associated with many horticultural and field crops, including most vegetables, fruit trees, grapevines, nuts, clovers and lucerne. They are found in all production areas in Australia. They cause knotting or galling of roots, and reduce vigour by interfering with root efficiency. Potential for damage is increased in light, sandy soils and poor growth may occur where young olive trees are planted into soil containing high levels of these nematodes. Uneveness and delaying of bloom have also been associated with these nematodes in California (M.V. McKenry, pers. comm.). Resistance has been reported against Meloidogyne incognita in cvv. Meashon and Tofany (Al Sayed and Abdel-Hameed, 1991), and against M. incognita and root rot in cvv. Krygula and Picual (Ghoneim et al., 1996). ‘Allegra’, a cultivar developed by the University of California, is reported to have resistance to multiple Meloidogyne spp. in the laboratory (McKenry, 1994).
Citrus nematode
This nematode is found in all Australian States especially in land previously planted to citrus and grapevines. It is particularly common along the River Murray and is not restricted to sandy soils. Tests conducted at the Plant Research Centre have shown levels of over 5000 Citrus Nematode females per 10g of olive roots from Riverland trees; such levels in citrus are considered to exceed damage thresholds justifying treatment with nematicides. Damage thresholds (for this nematode, and for other nematodes) in olives have not been determined, however, increased yields and flower development have been reported in California following application of nematicides to control Citrus Nematode (McKenry, 1994). In citrus, this nematode causes a slow decline and gives trees a twiggy appearance. Resistance has not been reported and land infested with this nematode is best avoided for olive production.
Root lesion nematode
Pratylenchus species reported to damage olives, P. vulnus and P. penetrans (McKenry, 1994), have been recorded in Australia on a wide range of crops including fruit and nut trees, grapevines, vegetables, roses and other ornamentals, and clover (McLeod et al., 1994). Records on olives in Australia are lacking, however, serious symptoms including stunting and branch dieback in young trees have been reported in California (McKenry, 1994). Parts of larger roots or whole small roots die from nematode feeding and then rotten and darken. Damage from root rot and wilt fungi is exacerbated in many crops by the presence of these nematodes. McKenry (1994) suggested that P. neglectus (a species common on cereals and other field crops in Australia) did not feed on olive roots.
Other nematodes
Various other nematodes, including Helicotylenchus, Xiphinema, Rotylenchulus, Hoplolaimus and Criconemella spp., have been recorded on olives in other countries but their potential for damage is unknown.
Nematode control
- If possible avoid planting in infested land.
- Test soil before planting to determine what nematodes are present and their levels (500g samples taken from the root zone, with roots if possible, can be sent to G. Walker, SARDI Plant Research Centre, Locked Bag 100 Glen Osmond SA 5064).
- Use clean planting material.
- If levels are high, fumigate soil before planting (eg methyl bromide, metham, Basamid granular ®), or bare fallow or crop to resistant cover crops for as long as possible to reduce levels. Strip fumigation can reduce costs but re-invasion from untreated areas can occur.
- Organic soil amendments have in some crops been shown to help plants tolerate nematode attack but may be inadequate against high levels.
- Post-planting nematicides such as Nemacur® have not been registered for use on olives.
Soil-borne fungal pathogens
Verticillium dahliae, cause of Verticillium Wilt in many crops including fruit and nut trees, vegetables, field crops and ornamentals, is found in all Australian States. In California (Teviotdale, 1994), this disease kills both young and mature trees, and it is likely that similar conditions prevailing in Australian production areas will see this disease assume increasing importance here as well. Symptoms described are collapse and death of one or more branches in the first hot weather of the season, with dead leaves and bloom remaining attached to branches, but without the vascular discolouration inside branches characteristic of the disease in many other crops (Teviotdale, 1994). The fungus can survive in soil for at least 14 years, and infection is favoured by cool, moist conditions.
Soil can be tested using selective agar media but such methods usually can test only small quantities of soil and are therefore not wholly reliable; rapid DNA-based tests are being developed at the Plant Research Centre, and larger volumes of soil can be tested using bait plants such as eggplants or tomatoes grown in pots. Avoid planting in land likely to be infected, particularly old vegetable ground. Soil fumigation can reduce levels but eradication is very difficult; methyl bromide/ chloropicrin mixtures and metham have been most commonly used against this fungus, and have generally given more consistent results than soil solarisation using clear plastic sheeting. ‘Ascalano’ is not resistant but reportedly tolerates the disease better than some other cultivars (Teviotdale, 1994). Once trees become infected there is no reliable method of control. For diagnosis, affected branches should be sent early in the season to a diagnostic laboratory for isolation of the fungus; summer heat can make isolation difficult by killing the fungus in the branches (but not in the roots).
An early record of Phytophthora cinnamomi on olives exists in Australia but there is little information on the significance of this record; this pathogen is well known as an important pathogen of other fruit trees such as avocados, as well as native plants. In California (Teviotdale, 1994), several other species including P. citricola and P. dreschleri are considered to be real or potential causes of Root and Crown Rot of olives. The latter 2 species have both been recorded in Australia. Phytophthora diseases are promoted by wet conditions and they are readily spread in planting stocks. Avoid planting in soil subject to flooding and keep irrigation emitters away from trunks. Systemic fungicides such as Ridomil® and Foli-R-Fos® commonly used in other crops against this disease have not been registered for use in olives.
In California (Teviotdale, 1994), Armillaria Root Rot caused by Armillaria mellea attacks olives. Another species of this fungus, A. luteo-bubalina is quite common in Australia but there is no information on its effects on olives. Other root rotting fungi such as Rosellinia necatrix are recognised pathogens of olives in other countries.
References
Al-Sayed, A.A. and Abdel-Hameed, S.H. (1991). Resistance and susceptibility of olives to Meloidogyne incognita and Rotylenchulus reniformis. Annals of Agricultural Science, Moshtohor 29:1221–6.
Cook, R.P. and Dube, A.J. (1989). Host-pathogen Index of Plant Disease in South Australia. Field Crops Pathology Group, South Australian Department of Agriculture. 142 pp.
Ghoneim, S.S.H., Abdel-Massih, M.I. and Mahmoud, F.A.F. (1996). Interaction between Root-knot Nematode and root rot on olive trees. Annals of Agricultural Science (Cairo) 41:445–61.
McKenry, M.V. (1994). Nematodes of olive. In ‘Olive Production Manual’ p.97–9. Ed. Ferguson, L., Sibbett, G.S., and Martin, G.C. University of California Division of Agriculture and Natural Resources, Oakland. Pub. 3353.
McLeod, R., Reay, F. and Smyth, J. (1994). Plant Nematodes of Australia Listed by Plant and by Genus. NSW Agriculture and RIRDC. 201 pp.
Morschel, J.R. (1961). Recorded Plant Diseases In and Outside Australia. Part 2 – Fruit and Edible Nut Crops. Commonwealth Department of Health Division of Plant Quarantine, Canberra 113 pp.
Teviotdale, B.L. (1994). Diseases of olive. In ‘Olive Production Manual’ p.107–9. Ed. Ferguson, L., Sibbett, G.S., and Martin, G.C. University of California Division of Agriculture and Natural Resources, Oakland. Pub. 3353.
