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Diseases of lucerne - 3: fungal root and crown diseases | AG0728 |
Rod Clarke, Knoxfield
November, 1999 |  |
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Rhizoctonia canker/root rot
Importance:
Rhizoctonia root canker or root rot, caused by the fungus Rhizoctonia solani can cause a slow thinning to the point where the stand is uneconomical within four years.
Symptoms:
Lucerne stands may appear patchy due to stunted and wilting plants growing among healthy plants. Affected areas may be circular or irregular in shape. There are no predominant leaf symptoms. The fungus attacks the base of the lateral roots, causing a brown to black round canker on the tap root. The cankers vary in size from 1 mm to 10 mm, and eventually girdle the whole tap root, causing the plant to wilt and die. The fungus also attacks the crown and stem bases of lucerne plants. Dark sunken elongated cankers form near the base of the stem, often girdling it and causing the leaves to yellow and wilt.
Biology:
Survival: The fungus carries-over as resting bodies, sclerotia, either in the soil or in plant debris.
Environmental conditions: High soil temperatures (25° to 30°C) and moisture favour disease development. Under flooding or irrigation, plant parts above the soil may be affected. Mechanical damage to roots and crowns and unfavourable soil conditions are also associated with this disease.
Dispersal: Under moist soil conditions the resting bodies, sclerotia, germinate and the fungus moves through the soil until it finds a nearby susceptible host.
Host Range: Rhizoctonia solani has a vast host range including: weeds, lucerne, red clover, white clover, subterranean clover, pasture grasses, grain legumes, cereals and oilseed crops.
No control measures are known.
Limit machinery movement through the stand to minimise damage to crowns.
Improving drainage may be useful.
Phytophthora root rot
Importance:
Phytophthora root rot, caused by the fungus Phytophthora megasperma can cause rapid thinning of stands particularly if they are flood irrigated. Seedlings are highly susceptible to Phytophthora root rot.
Symptoms:
Phytophthora megasperma causes a yellow-brown soft rot, which later turns black, on the tap root, which may girdle it and extend into the crown. The leaves, particularly the lower ones, are yellow or bleached. Regrowth of affected plants after cutting is very poor. Under dry conditions the plant may grow secondary surface roots from the tap root stub which will enable the plant to survive, although growth will be retarded.
Biology:
Survival: Phytophthora megasperma can carryover in the soil as zoo spores or as fungal growth (mycelium) on infected plant tissue.
Environmental conditions: Phytophthora root rot is favoured by wet, poorly drained soil conditions, during periods of excessive rainfall and/or flood irrigation, and high soil temperatures (20°C).
Dispersal: Irrigation water or free flowing water after heavy rain, can carry zoospores that have been released from a spore producing sack (sporangia) formed on infected roots.
Host Range: Phytophthora megasperma occurs on lucerne, carrot, pine trees, soybean crucifer vegetables and Melilotus spp.
Control:
Use resistant cultivars.
Improve irrigation management and surface drainage to reduce excess water lying in bays.
Fusarium root rot
Importance:
Root rot caused by the fungi Fusarium spp. are common and may infect plants at any stage of growth. Losses may vary from minor to very severe.
Symptoms:
The first evidence of Fusarium Wilt is a stunting and rapid wilting of the stems on one side of the plant. Leaves and stems appear light green or bleached with the lower leaves a light pink colour. Stem tops may wilt during hot weather or when the plant is under moisture stress and then recover in the cool of the night or after rain/irrigation.
Characteristic reddish-brown streaks occur within the water conducting tissue (stele) of the roots. The outer tissue of the roots remains unaffected. These streaks may only involve part of the root, but later all the conducting tissue may be discoloured. In dying plants the crown and tap root are completely rotted.
Biology:
Survival: Fusarium spp. survive in the soil as spores, or as fungal mould in live and dead plant tissue. Soils may remain infested for many years.
Environmental conditions: This disease is favoured by high soil temperatures, and will affect plants that have been set back by low soil fertility, insect damage, mechanical damage or other diseases.
Dispersal: The Fusarium spp. fungi may be spread by wind carried infected plant debris and soil, or by infected soil adhering to machinery and stock feet.
Host Range: Fusarium spp. fungi have a very wide host range including; weeds, pasture legumes and grasses, cereals, grain legumes and oilseed crops.
Control:
Avoid overgrazing to limit stress on plants.
Maintain adequate soil fertility and pH.
Crop rotations help limit the build up of fungi in the soil.
Pythium root rot/damping off
Importance:
Pythium root rot/damping off, caused by the fungi Pythium spp., pre and post emergence, can result in thin unproductive stands. Rotting of the lateral or fibrous roots in older plants can also lead to thinning in patches to a limited extent.
Symptoms:
Infected seeds and seedlings are soft and mushy. More frequently the fungi infect the base of the stem at about the soil line after they emerge. The basal tissue develops a brown discolouration soft rot. The leaves redden and seedlings fall over or become stunted. Under moist humid conditions, seedlings may survive by developing secondary roots above the lesion.
Biology:
Survival: Pythium spp. survive in the soil or in infected plant debris as oospores. These oospores produce a sack, or sporangium, in which zoospores are formed.
Environmental conditions: High soil moisture and low temperatures favour disease development. These conditions also inhibit plant development, thus making plants susceptible to infection.
Dispersal: Zoospores, when released from the sporangium swim through free water to infect roots or seeds.
Host Range: Pythium spp. have a very wide host range including; weeds, pasture legumes and grasses, cereals, grain legumes and oilseed crops.
Control:
Treat seed with a fungicide seed protectant.
Improve irrigation management and surface drainage to reduce excess water lying in bays.
Do not sow too thickly, as this can encourage the rapid spread of the disease.
Stagonospora crown rot
Importance:
Stagonospora crown rot, caused by the fungus Stagonospora meliloti, tends to occur in stands over two years old, and causes a slow decline over three or four years, but incidence is low.
Symptoms:
Severely affected plants are stunted, but there is little yellowing of the foliage. Within the roots there is a reddish speckling and often a flecking, in areas where the fungus is slowly moving through the root. This discolouration later turns to a dry brown rotted area. The root rot spreads from the crown and into the lower sections of the stems. Leaf spotting is also known to be caused by Stagonospora meliloti.
Biology:
Survival: The fungus caries-over on infected plant debris in fruiting bodies, pycnidia.
Environmental conditions: Plants under moisture, nutritional, insect or disease stress are most susceptible to this disease.
Dispersal: Under wet conditions, spores are released from the fruiting bodies, pycnidia, and spread by rain splash or irrigation water to other plants.
Host Range: Stagonospora meliloti occurs on lucerne and medics.
Control:
Avoid undue stress (moisture and nutrition) on stand.
Avoid excessive damage to stand through overgrazing or machinery traffic.
Anthracnose/crown rot
Importance:
Anthracnose/crown rot, caused by the fungus Colletotrichum trifolii occurs sporadically, but when severe will kill shoots and crown, thus thinning the stand.
Symptoms:
The first signs may be numerous dead stems scattered through the stand. Stem lesions may vary from irregularly shaped blackened to large sunken, oval to diamond shaped areas. These lesions become bleached in the centre and develop small, pinhead sized fruiting bodies. The lesions and fruiting bodies are generally found close to the crown, but may extend up the stem, 80 - 100 mm.
On the crown, the rotted area is yellow to tan, often with a narrow band of reddish-brown tissue along the border of the advancing rot. In an advanced stage the rotted areas become a blue-black colour which can extend from the base of the tillers down the taproot for up to 80 mm.
Biology
Survival: The fungus carries-over on infected plant debris, on stems and in the living crowns.
Environmental conditions: Infection is favoured by warm moist weather.
Dispersal: Spores from the fruiting bodies are rainsplash or dew-washed to nearby plants.
Host Range: Colletotrichum trifolii occurs on lucerne and red clover.
Control:
Use resistant cultivars.
Clean hay making machinery of debris before first cut and between paddocks.
Allow two to three years between lucerne crops to allow for the break down of infected debris and crowns.
Sowing rates should not be too dense; this will minimise humidity within the stand.
The advice provided in this publication is intended as a source of information only. Always read the label before using any of the products mentioned. The State of Victoria and its employees do not guarantee that the publication is without flaw of any kind or is wholly appropriate for your particular purposes and therefore disclaims all liability for any error, loss or other consequence which may arise from you relying on any information in this publication.
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