Hot weather leads to char­coal rot warn­ing

The Northern Star - Northern New South Wales Rural Weekly - - Front Page -

THE dis­ease of char­coal rot has been build­ing up in soy­bean crops in north­ern NSW due to the hot, dry con­di­tions ex­pe­ri­enced dur­ing the past few sum­mers.

Pad­dock se­lec­tion is crit­i­cal to min­imise the im­pact of this dis­ease and ro­ta­tion with non-host crops is ad­vised.

DPI re­search agron­o­mist Na­talie Moore had a mes­sage for grow­ers: “It is timely to think about soy­bean pad­dock ro­ta­tion, es­pe­cially since char­coal rot has built up in re­cent sum­mers.

“Back-to-back soy­bean plant­ing is a high risk if the dis­ease was se­vere in the pre­vi­ous sea­son.”

Char­coal rot of soy­bean is caused by the soil-borne fun­gus Macrophom­ina phase­olina, a pathogen with a wide host range, in­clud­ing all sum­mer crops but par­tic­u­larly soy­bean, sorghum, sun­flower, mung­bean, maize and cot­ton, many hor­ti­cul­tural crops and trees, and also weeds.

In Aus­tralia, more than 80 host plants have been iden­ti­fied and the fun­gus has been found in all main­land states and ter­ri­to­ries.

The dis­ease is the most com­mon in soy­beans in Aus­tralia, and can cause the pre­ma­ture death of en­tire crops.

Ev­i­dence sug­gests that in­fec­tion oc­curs when the seedlings are grow­ing ac­tively and that the pathogen re­mains la­tent inside roots un­til the plants are stressed.

Phys­i­o­log­i­cal stresses as­so­ci­ated with flow­er­ing com­bine with an ex­ter­nal stress, most fre­quently high tem­per­a­tures and/or low soil mois­ture, to stim­u­late the pathogen into ac­tiv­ity. Stress from wa­ter­log­ging fol­lowed by hot weather, and per­haps stem in­sect in­jury, can pre­dis­pose plants to the rot.

Dur­ing this stage, M. phase­olina pro­duces a plant toxin which, to­gether with a plug­ging of the wa­ter con­duct­ing tis­sue, causes rapid wilt­ing and ul­ti­mately plant death. As in­fected plants die mi­croscle­rotes com­posed of tightly com­pacted fun­gal strands are pro­duced in abun­dance in the roots and lower stems.

Char­coal rot symp­toms ap­pear on soy­bean plants usu­ally at and af­ter flow­er­ing, al­though hot, dry weather im­me­di­ately af­ter emer­gence can kill seedlings.

From flow­er­ing on­wards, in­di­vid­ual plants scat­tered across a pad­dock sud­denly wilt, with leaves dy­ing rapidly but re­main­ing at­tached to the peti­oles. In fur­row-ir­ri­gated crops, plants at the head ditch end often wilt first and plants grow­ing in lighter soil tend to wilt first.

The stems of in­fected plants turn from green to a light yel­low-tan colour, later be­com­ing brown. On oc­ca­sion, in­fected plants will dis­play a dark brown le­sion from ground level up the stem, end­ing in a gra­da­tion from dark brown to yel­low into the green stem above the le­sion.

A di­ag­nos­tic symp­tom of char­coal rot in the early stages of de­vel­op­ment is an or­ange dis­coloura­tion of the tis­sue just be­low the sur­face of the stem, com­monly called “or­ange bark disor­der”.

Char­coal rot-in­fected plants al­most al­ways die be­fore ma­tu­rity, and af­ter death the stems usu­ally turn an ashen grey colour with minute black specks on the sur­face. When the dead tap roots and basal stems are split this “pep­per” symp­tom is also read­ily ap­par­ent.

The specks are the sur­vival struc­tures of the dis­ease, form­ing a hard rind at the sur­face, en­abling them to sur­vive in the soil for years.

LOAD OF ROT: Early symp­toms of char­coal rot in north­ern NSW.

De­struc­tion caused by char­coal rot and drought stress at In­verell. PHO­TOS: N MOORE, NSW DPI

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