Good soil struc­ture lim­its dis­ease

Can­ter­bury re­search com­pared pota­toes grown in dis­eased and clean fields with good and poor soil struc­ture.

NZ Grower - - Potatoes Nz Inc. - By Heather Chalmers

“These soil qual­ity mea­sures could help farm­ers to make more in­formed de­ci­sions about which pad­docks could po­ten­tially yield bet­ter for potato pro­duc­tion.”

The con­tin­ued pres­ence of soil and seed-borne dis­eases in potato crops can be partly mit­i­gated by sow­ing crops in pad­docks with good soil struc­ture, Plant & Food Re­search work shows.

Pre­vi­ous potato yield gap work from 2012-15 iden­ti­fied that soil-borne dis­ease and soil phys­i­cal qual­ity were two ma­jor yield lim­it­ing fac­tors. In 2015, a Sus­tain­able Farm­ing Fund (SFF) project in Can­ter­bury was set up to help quan­tify these re­sults. Ac­cord­ing to Plant & Food re­search as­so­ciate at Lin­coln, Steve Del­low, year two of the project fo­cused on crop­ping his­tory and what im­pact that had on potato yields. For this re­search, 15 com­mer­cial fields were se­lected that were to be planted in two cul­ti­vars, Rus­set Bur­bank or In­no­va­tor. Fields were cho­sen based on four cat­e­gories. A dis­eased field was de­fined as one which had grown pota­toes once within the last 10 years. Clean pad­docks were those that had not grown pota­toes for more than 10 years. These were fur­ther bro­ken down into pad­docks with good or poor soil struc­ture. Pad­docks with a good soil struc­ture had a crop­ping his­tory that was pre­dom­i­nantly restora­tive, no­tably long-term pas­ture. Pad­docks with poor soil struc­ture had been con­tin­u­ally cropped.

In terms of soil-borne dis­ease the trial showed that crop­ping his­tory in­flu­enced the preva­lence of Rhi­zoc­to­nia stem canker.

“If you had grown pota­toes in the last 10 years you had an in­creased chance of hav­ing Rhi­zoc­to­nia stem canker,” he said.

“Com­ing out of long-term pas­ture also led to an in­creased in­ci­dence of stem canker. For Spon­gospora (the pathogen caus­ing pow­dery scab and root galls) the trend was the same.”

De­spite in­creased dis­ease in­ci­dence, no yield re­duc­tion was seen.

“Mar­ketable yield was greater, at an av­er­age 10 tonne higher, from pad­docks that had come out of longterm pas­ture com­pared with land that had been an­nu­ally cropped.”

The se­cond part of the re­search project looked at soil phys­i­cal qual­i­ties to ex­plain this yield dif­fer­ence. Each pad­dock was given a 10-year crop his­tory score, based on how restora­tive it was and how much or­ganic mat­ter was re­turned to im­prove the soil struc­ture. An in-field visual as­sess­ment was also made of soil struc­tural qual­ity and how favourable this was for root pen­e­tra­tion.

“With those two meth­ods we got a rel­a­tively good cor­re­la­tion with yield,” he said.

“As you move from a non-restora­tive, an­nu­ally cropped pad­dock his­tory to a more per­ma­nent pas­ture and soil struc­ture im­proved, you got an in­crease in yield. This was more so for Rus­set Bur­bank, which was more sen­si­tive to poorly struc­tured soils.

“Based on the anal­y­sis, we can say that about half of the yield vari­abil­ity can be ac­counted for by soil struc­ture.

“Even though we had bet­ter soil struc­ture from hav­ing more per­ma­nent pas­ture in the crop ro­ta­tion we still have more dis­ease. How­ever, as the plants have more ac­cess to wa­ter, air and nu­tri­ents they can de­fend them­selves

more against the pres­ence of that dis­ease.

“These soil qual­ity mea­sures could help farm­ers to make more in­formed de­ci­sions about which pad­docks could po­ten­tially yield bet­ter for potato pro­duc­tion.”

Potato seed line health

In re­lated re­search from 2016-18, Alex Michel and his team mon­i­tored the health of seed potato lines. In year one, six com­mer­cial Agria and five In­no­va­tor lines were mon­i­tored and com­pared for in-sea­son health, com­pared with fol­low­ing daugh­ter main crops.

The seed was also grown on as whole seed in dis­ease-free pot­ting mix in a con­trolled, dis­ea­sein­duc­ing en­vi­ron­ment to es­ti­mate the po­ten­tial pres­ence of seed­borne pathogens. In­ci­dence and sever­ity of Rhi­zoc­to­nia stem canker and Spon­gospora root galling was low in the seed crops and low to mod­er­ate in the glasshouse plants. This trend was noted in sub­se­quent repli­cated tri­als and in sev­eral com­mer­cial crops na­tion­wide, us­ing seed col­lected from the mon­i­tored lines. How­ever, one daugh­ter com­mer­cial crop be­came highly dis­eased, as it was stressed by drought and flood con­di­tions in the Manawatu.

These re­sults show that if seed­borne in­ocu­lum can be min­imised through the sup­ply chain, the im­pact on well-man­aged crops can be min­i­mal, Michel said.

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