Farm Forestry New Zealand

PESTS AND DISEASES OF FORESTRY IN NEW ZEALAND

Damping-off

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Formerly known as the Forest Research Institute, Scion has been a leader in research relating to forest health for over 50 years. The Rotorua-based Crown Research Institute continues to provide science that will protect all forests from damage caused by insect pests, pathogens and weeds. The information presented below arises from these research activities.

From Forest Pathology in New Zealand No. 16. Nursery diseases.
Based on Margaret Dick and AL Vanner  (1986),
Revised by MA Dick (2008).

Causal organisms
Many soil-inhabiting fungi have been associated with damping-off. In New Zealand the most important are:
Cylindrocladium scoparium Morgan,
Fusarium spp.,
Phytophthora spp.,
Pythium spp.,
Rhizoctonia solani Kuhn


Fig. 10 – Small Pinus radiata seedlings with collapse of the succulent stem caused by Phytophthora sp.

Type of injury
Seedling mortality.

Diagnostic features

• Bare spaces in nursery beds.
• Small seedlings lying on the ground after collapse of succulent stems (Fig. 10).

Hosts
Small seedlings of all plants are susceptible to attack by damping-off fungi.

Distribution
Throughout New Zealand.

Disease development
All the tissues of very young seedlings are susceptible to infection by non-specific parasites. These diseases are more prevalent in moist soils and wet weather, hence the name "damping-off". There are two types of damping-off:
(a) Pre-emergence damping-off is recognised by bare spaces in the rows of seedlings caused by some germinating seeds rotting before they break through the soil. Though a sparse crop may be due to low viability of seed, it is more frequently a result of soil fungi attacking the germinating seeds.
(b) Post-emergence damping off is the collapse and withering of seedlings soon after they have emerged from the soil. This may be due to fungal attack on the stem or roots or both.
Damping-off fungi grow freely from the diseased tissue back into the soil. All damping-off fungi normally live on dead plant material in the soil and their continued development is enhanced by the nutrient supply provided by the dead seedlings.

Cylindrocladium scoparium has been recorded less frequently in forest nurseries than the other damping-off pathogens, but can cause significant losses. In addition to below-ground attack, C. scoparium may also attack the succulent stem or first leaves. This occurs under conditions of high humidity following excessive rain (or over-watering), and there may be rapid spread of the fungus from seedling to seedling. Spores may be produced on the dead stems giving a powdery appearance to the host. When environmental conditions are unfavourable for growth, sclerotia (small dense masses of fungal tissue) are formed and these remain dormant in the soil until conditions improve.

Infection by Fusarium species can occur over a wide soil moisture range, but the optimum soil temperature is between 20 and 30^oC, which is generally higher than for other damping-off fungi. Thick-walled spores (chlamydospores) are formed under adverse environmental conditions and remain dormant in the soil until conditions are suitable for germination.

Phytophthora and Pythium species are closely related and are generally considered to be the most destructive and the most prevalent of the damping-off micro-organisms. Infection occurs when the spores come in contact with susceptible root tissue, germinate, and penetrate the root wall. As the spores are mobile, moving free in soil water between the soil particles, high soil moisture content is needed for spore release and dispersal.

As Rhizoctonia solani is strongly aerobic, attack by this fungus is generally confined to the upper 1.5 cm of soil and to stems in the saturated air just above the soil. Abundant soil moisture inhibits the activity of this fungus in the soil, although at these times it may grow over the soil surface attacking emerging seedlings. Rhizoctonia does not produce spores but forms abundant sclerotia. In very acid soils, R. solani tends to predominate over other damping-off fungi.

Economic importance
Pre-emergence losses from fungal attack are difficult to assess because the damage is hidden, and poor germination may result from other causes. Even the effects of post-emergence damping-off may be confused with those caused by other damaging agencies, such as sun scorch, frost, or drought. No seedling can be regarded as resistant to damping-off but those that germinate quickly and grow fast are less likely to be affected.
Given the appropriate environmental conditions, losses resulting from damping- off can be extremely high.

Control
Damping-off diseases are best kept in check by following sound nursery practices. Damping-off is less likely to occur where seedlings are not placed under stress. Stratification of seed ensures even germination and reduces the period of time over which the crop is susceptible. Where chemical control of the disease is considered desirable, the following treatments have been shown to give good results.

Pre-sowing application of Terrazole 35wp (350g/kg etridiazole) at a rate of 130kg/ha followed by rotary hoeing to a depth of 15cm. This fungicide is effective against species of Phytophthora and Pythium.

Pre-sowing application of Basamid Granular (970g/kg dazomet) at a rate of 500kg/ha followed by rotary hoeing to a depth of 15cm. This chemical is a soil ‘sterilant’ and kills a wide range of fungal species and soil organisms including nematodes. Germination trials with cress seed need to be undertaken prior to seed sowing to ensure that complete dissipation of the chemical has occurred.

A pre-emergence application of metalaxyl (available as Ridomil products) at a rate of 1-4 kg/ha a.i. in 200-300 litres of water per hectare has been recommended. Where possible, apply to damp ground. Use the higher rate for soils with a high organic content and on beds with a previous history of root rot problems. The half-life of metalaxyl is strongly influenced by soil type and has been reported to range from 82 days to less than one day.  Furthermore, degradation of metalaxyl can be accelerated in soils with a history of metalaxyl usage. About 10 mm of rain or irrigation is required to move the fungicide into the soil.

A post-emergence drench of captan at a rate of 1.2 kg/ha a.i. in 500 to 1,000 litres of water per hectare has been recommended for controlling damping off in forest nurseries. E.g. Captan 8OW or (Merpan 800g/kg captan) at a rate of 1.4kg/ha.

Monthly applications of phosphorous acid (e.g. Foli-R-Fos, Foschek) at a rate of 2.6 kg/ha a.i. from December, or fortnightly from February gave good control of Phytophthora root disease in a recent trial.

Seed treatment prior to sowing with a fungicide such as Aliette WG (800 g/kg fosetyl aluminium), Aliette Super (528g/kg fosetyl aluminium. 172g/kg thiram. 129g/kg thiabendazole), Thiram, Apron C70 SD (350g/kg metalaxyl, 350g/kg captan), or Captan 80W may control damping-off. These fungicides have been recommended for use as seed dressings to control damping off fungi on seed of other crops.

Bibliography
Bassett, C. 1961: Some soil-borne fungi in forest nurseries. New Zealand Science Review 19: 24.
Phillips, D.H.; Burdekin, D.A. 1982: "Diseases of forests and ornamental trees." Macmillan Press Ltd. Pp. 52-55.

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(Scion is the trading name of the New Zealand Forest Research Institute Limited.)