Colletotrichum Crown Rot or Anthracnose
It has been estimated in studies in the United States that 24% of the forage and 90% of the seed yield of lucerne are lost annually through disease. Although no data has been gathered, losses of these magnitudes could also be occurring in southern Queensland.
Diseases reduce yield, forage quality and persistence, cause defoliation, increase damage from environmental stresses, and reduce the efficient use of water and nutrients. The importance of each disease may vary among districts as well as seasons because of the varying conditions under which the crop is grown.
More than thirty lucerne diseases have been recorded in Queensland — some rare, some common and some causing significant losses. They can be divided into three main categories — seedling diseases, leaf and stem diseases, and root and crown diseases.
Key to disorders lethal to lucerne in Queensland
1. Plants with numerous fine shoots; leaflets small, rounded and puckered………………………legume little leaf
Plants without numerous fine shoots; leaflets not rounded or puckered……………………………………………………….2
2. Lesions or cankers on crown and/or roots ………………………………………………………………………………………………..3
Lesions absent from crown and/or roots…………………………………………………………………………………………………………4
3. Roots lesions up to 5 cm long, surrounding taproot at or just below ground level; internal
tissue brown, with discolouration extending up to 5 cm beyond margin of surface lesion
phytophthora root rot Round, fissured cankers on taproot and crown; cankers surround
the taproot but internal discolouration is limited to the region below canker; branch roots
rot where they emerge from taproot……………………………………………..rhizoctonia root, crown or stem canker
4. External mycelium clearly visible to naked eye on crown or taproot………………………………………………………….5
External mycleium absent………………………………………………………………………………………………………………………………..6
5. Mycelium violet, completely covering the taproot and/or crown………………………………………….violet root rot
Mycelium white, covering crown branches and upper portion or taproot; brown sclerotes,
1-2 mm in diameter, often present…………………………………………………………..sclerotium blight/ sclerotinia rot
6. Taproot surface shrivelled and fissured, no internal discolouration………………………………………..water stress
Internal tissues of crown and taproot discoloured………………………………………………………………………………………….7
7. Tunnels, up to 5 mm wide, in internal tissues of crown branches, often
extending into taproot; insect larvae present in tunnels; tissue surrounding tunnel
dry-rotted, brown and shredded…………………………………………………………………………………….crown borer damage
8. Taproot with cavities and secondary roots missing or damaged white fringed weevil
Insect tunnels absent from internal discoloured tissue…………………………………………………………………………………..9
9. Internal crown tissue bluish-black, with discolouration often extending into
taproot……………………………………………………………………………………………………………………..colletotrichum crown rot
Bluish-black discolouration absent from internal crown tissue……………………………………………………………………….10
10. Red flecking at the margin of an internal wedge-shaped, dry-rotted portion of the crown, often
extending into the-taproot…………acrocalymma crown and root rot or stagonospora crown and root rot
Vascular tissue of the taproot discoloured, without red flecking…………………………………………………………………..11
11. Roots are bleached white, and soft, with a light-yellow discolouration of vascular
cylinder; root tips blackened……………………………………………………………………………………………………….waterlogging
Roots firm, with a dark-brown discolouration in the vascular tissue of the taproot, often extending into crown
branches and tillers………………………………………………………………………………………………………………………fusarium wilt
Violet Root Rot
Using the keys
1. Read through the list of descriptions until you find one that matches your requirements.
2. Note the corresponding disorder/leaf disease on the right-hand side of the key. If a number is provided
instead, go to that number in the key, and read the description.
3. Repeat the process until an identification can be determined.
Key to common lucerne leaf diseases in Queensland
1. Leaflets distorted…………………………………………………………………………………………………………………………………………2
Leaflets not distorted……………………………………………………………………………………………………………………………………….3
2. Leaflets small, puckered and pale green, plants with spindly shoots………………………………….witches broom
Leaflets twisted and with yellow areas, violet downy growth on lower surface……………………..downy mildew
3. Leaflets with large yellow areas………………………………………………………………………………………………………………….4
Leaflets with distinct spots………………………………………………………………………………………………………………………………5
4. Leaflets with violet downy growth on lower surface…………………………………………………………….downy mildew
Leaflets without downy growth, yellowing in a mottled pattern………………………………………alfalfa mosaic virus
5. Spots white, mainly near edges………………………………………………………………………………..potassium deficiency
Spots light tan-black, not restricted to edges………………………………………………………………………………………………..6
6. Spots consisting of red-brown pustules easily rubbed off……………………………………………………………………..rust
Spots not as above………………………………………………………………………………………………………………………………………….7
7. Spots mostly larger than 3 mm and light coloured……………………………………………………………………………………..8
Spots small (1-3 mm)……………………………………………………………………………………………………………………………………….9
8. Spots oval-elongate with bleached centres and distinct dark borders………………………….stemphylium leaf spot
Spots irregular, light red-brown and with a diffuse yellow halo……………………………………………..cercospora leaf spot
9. Spots watersoaked at first, later with a tan centre and yellow halo, causing affected
areas to become papery……………………………………………………………………………………………………..bacterial leaf spot
Spots dark and not water-soaked initially……………………………………………………………………………………………………..10
10. Spots consisting of a brown-black raised area surrounded by a yellow halo common leaf spot
Spots without a raised area in centre…………………………………………………………………………………………………………….11
11. Spots peppery, later with a yellow halo, coalescing to cause extensive leaf death……………………………………………………………………………………………………………………..leptosphaerulina leaf spot
Spots black without a halo, also on stems and petioles………………………………………….black leaf and stem spot
Using the keys
1. Read through the list of descriptions until you find one that matches your requirements.
2. Note the corresponding disorder/leaf disease on the right-hand side of the key. If a number is provided
instead, go to that number in the key, and read the description.
3. Repeat the process until an identification can be determined.
.
