Prod­uct groups

NZ Grower - - Contents -

KEY POINTS:

• To date this project has not iden­ti­fied any new con­trol strate­gies which are more ef­fec­tive than com­monly used treat­ments.

• A repli­cated trial was set up in a com­mer­cial crop in South Can­ter­bury with the potato cul­ti­var Rus­set Burbank. The trial site had been in pota­toes pre­vi­ously, so dis­ease pres­sure was likely to be high.

• Rhi­zoc­to­nia stem canker and Spon­gospora root galling and tu­ber pow­dery scab were the pre­dom­i­nant dis­eases found on the sam­pled plants. There was no sig­nif­i­cant dif­fer­ence be­tween any of the treat­ments for the con­trol of Rhi­zoc­to­nia, apart from for­ma­lin, which in­creased rhi­zoc­to­nia stem canker sever­ity by 12%.

• Spon­gospora root gall sever­ity and in­ci­dence in­creased as the sea­son pro­gressed, how­ever there were no dif­fer­ences be­tween treat­ments.

• Plants grown from for­ma­lin treated seed de­vel­oped more se­vere root galling than those from un­treated seed. For­ma­lin re­duced yields by 6 t/ha.

• There was no sig­nif­i­cant yield dif­fer­ence be­tween the con­trol, Amis­tar × 2 rate, Pin­na­cle, Mon­ceren or Mon­ceren × 2 rate treat­ments (yields range 73–82 t/ha).

• Yields were re­duced by the treat­ments Maxim, Mon­ceren × 2 rate + Amis­tar × 2 rate and Ne­bi­jin (av­er­age 66 t/ha).

• No fungi­cide treat­ment in­creased yield in this trial, or in the three tri­als in the pre­vi­ous two sea­sons.

• The lack of ef­fi­cacy of fungi­cides is a ma­jor con­cern, par­tic­u­larly if they are re­duc­ing yields. There is a sig­nif­i­cant cost as­so­ci­ated with ap­ply­ing fungi­cides and if, at a num­ber of sites in New Zealand, they are pro­vid­ing no con­trol and po­ten­tially re­duc­ing yield then the rea­son needs to be un­der­stood. IN­TRO­DUC­TION AND METH­ODS

In New Zealand potato crops, con­trol of the two dom­i­nant soil-borne pathogens, Spon­gospora sub­ter­ranea and Rhi­zoc­to­nia solani, has re­mained dif­fi­cult, de­spite the wide range of fungi­cides and pes­ti­cides that are reg­is­tered for con­trol­ling them.

The sever­ity of seed-borne and soil-borne dis­eases varies by site and sea­son. This up­date pro­vides fur­ther in­for­ma­tion from one high-risk site in South Can­ter­bury.

This trial is the fourth over three sea­sons. In a trial con­ducted in the 2015–16 sea­son test­ing seven fungi­cides, only one (Ne­bi­jin) was re­ported to have any ef­fect, low­er­ing pow­dery scab in­ci­dence and sever­ity at har­vest. This work was con­tin­ued in the 2016–17 sea­son, and ad­di­tional treat­ments were tested in or­der to iden­tify po­ten­tial new strate­gies for dis­ease man­age­ment.

A repli­cated trial was set up in a com­mer­cial potato crop at Lev­els, South Can­ter­bury, with the cul­ti­var Rus­set Burbank (planted 1 Novem­ber 2016). The trial site had been in pota­toes six years ago so dis­ease pres­sure was likely to be high.

The chem­i­cal treat­ments were ap­plied ei­ther directly to the seed tu­bers or as in-fur­row sprays at plant­ing, prior to clos­ing the fur­rows. Eight prod­ucts were in­ves­ti­gated, both with and with­out for­ma­lin, to­talling 16 treat­ments. Stan­dard crop man­age­ment was un­der­taken by the grower for the re­main­der of the sea­son. Dis­ease as­sess­ments on the un­der­ground stems and tu­bers were car­ried out at two crop growth stages (full canopy, 10 weeks af­ter plant­ing and late canopy, 14 weeks af­ter plant­ing) and at har­vest (tu­bers only). A final yield as­sess­ment based on mar­ketable tu­bers (t/ha of tu­bers >65 mm) was car­ried out at crop ma­tu­rity.

RE­SULTS

Dif­fer­ent dis­eases were de­tected on stems and tu­bers from the har­vested plants, but Rhi­zoc­to­nia stem canker and Spon­gospora root galling and tu­ber pow­dery scab pre­dom­i­nated.

There was no sig­nif­i­cant dif­fer­ence be­tween any of the treat­ments for the con­trol of stem canker, apart from for­ma­lin, which in­creased sever­ity by 12% (P = 0.002). Stem canker sever­ity de­vel­oped through the sea­son with 25% stem cov­er­age (av­er­age of all treat­ments) at full canopy, in­creas­ing to 36% at late canopy. This fol­lows the typ­i­cal pat­tern of dis­ease pro­gres­sion in many New Zealand potato crops.

The na­ture of Rhi­zoc­to­nia symp­toms ob­served on the potato plants may have re­flected the con­tri­bu­tion of both the seed-borne and soil­borne forms of this pathogen. The ap­pear­ance of symp­toms con­sis­tent with soil-borne in­fec­tion was ev­i­dent (Fig­ure 1, Fig­ure 2), in­di­cat­ing that the pre­vi­ous crop­ping se­quence favoured the sur­vival of Rhi­zoc­to­nia in the field. Seed-borne dis­ease in­ci­dence was less than soil-borne dis­ease in­ci­dence, and not re­lated to the treat­ments, suggest­ing that the seed (whole) may have been only lightly in­fected with the dis­ease.

Spon­gospora root gall sever­ity and in­ci­dence in­creased as the sea­son pro­gressed, and plants from for­ma­lin

treated seed de­vel­oped more se­vere root gall symp­toms than those that were not for­ma­lin treated. At the final as­sess­ment there was a small dif­fer­ence in pow­dery scab sever­ity be­tween the for­ma­lin treat­ments, where the use of for­ma­lin re­duced the sever­ity score from 0.45 to 0.38, where a score of 1 = 5% cov­er­age. >

For mar­ketable yield, there was no ev­i­dence of a treat­ment in­ter­ac­tion be­tween for­ma­lin seed tu­ber treat­ment and fungi­cide treat­ment. There was no sig­nif­i­cant dif­fer­ence in yield be­tween the con­trol, Amis­tar® × 2 rate, Pin­na­cle®, Mon­ceren®, Mon­ceren® × 2 rate (yield range 73-82 t/ha) treat­ments. Yield was sig­nif­i­cantly less with the Maxim®, Mon­ceren® × 2 rate + Amis­tar® × 2 rate and Ne­bi­jin® (yield range 61-68 t/ha). In this trial, the use of for­ma­lin de­creased the mar­ketable yields by 6 t/ha.

All treat­ments that ex­ceeded la­bel rate had tu­bers tested for residues. None of these ex­ceeded New Zealand residue lev­els.

DIS­CUS­SION

Al­though Rhi­zoc­to­nia and Spon­gospora dis­eases were wide­spread (and in some cases se­vere) in the trial area, none were con­trolled by any of the chem­i­cal treat­ments. In some cases, yield was re­duced by these treat­ments.

These re­sults are com­pa­ra­ble to find­ings from sim­i­lar tri­als con­ducted in the pre­vi­ous two sea­sons. For­ma­lin treat­ment of seed tu­bers is be­com­ing more pop­u­lar with potato grow­ers, but re­sults from the present trial show that this treat­ment may also cause yield re­duc­tions. To date this project has not iden­ti­fied any new con­trol strate­gies which are more ef­fec­tive than com­monly used treat­ments. Re­sponses to fungi­cides may need eval­u­a­tion at a wider range of sites and un­der dif­fer­ent dis­ease pres­sures.

Ac­knowl­edg­ments

Thanks to Plant & Food Re­search for pro­vid­ing ex­per­tise with dis­ease as­sess­ments, data analysis and in­ter­pre­ta­tion of re­sults, and to the Foun­da­tion for Arable Re­search (FAR) team who car­ried out es­tab­lish­ment of the trial and yield as­sess­ments. Mor­gan Bowles pro­vided land, and helped to plant the trial.

Dis­claimer: This pub­li­ca­tion is in­tended to pro­vide ac­cu­rate and ad­e­quate in­for­ma­tion re­lat­ing to the sub­ject mat­ters con­tained in it. It has been pre­pared and made avail­able to all per­sons and en­ti­ties strictly on the ba­sis that Pota­toes New Zealand In­cor­po­rated, the Foun­da­tion for Arable Re­search, their re­searchers, au­thors and con­trac­tors are fully ex­cluded from any li­a­bil­ity for dam­ages aris­ing out of any reliance in part or in full upon any of the in­for­ma­tion for any pur­pose. No en­dorse­ment of named prod­ucts is in­tended nor is any crit­i­cism of other al­ter­na­tive, but un­named prod­uct.

Pota­toes New Zealand has taken rea­son­able steps and ex­er­cised skill, care and dili­gence in pro­duc­ing this fact sheet to meet the re­quire­ments of grow­ers. How­ever there is no im­plicit or ex­pressed war­ranty that the in­for­ma­tion con­tained is free from er­ror or omis­sion. Pota­toes New Zealand In­cor­po­rated does not ex­pressly or oth­er­wise en­dorse the prod­ucts pro­moted in this fact sheet. Nei­ther Pota­toes New Zealand In­cor­po­rated, the Foun­da­tion for Arable Re­search, nor any of their em­ploy­ees or con­trac­tors shall be li­able for any cost (in­clud­ing le­gal costs), claim, li­a­bil­ity, loss, dam­age, in­jury or the like, which may be suf­fered or in­curred as a direct or in­di­rect re­sult of the reliance by any per­son on any in­for­ma­tion con­tained in this fact sheet.

▴ Ta­ble 1. Treat­ments, their ac­tive in­gre­di­ents, tar­get dis­ease and ap­pli­ca­tion meth­ods (ei­ther ap­plied to the potato seed or in-fur­row at plant­ing) as­sessed in South Can­ter­bury in the 2016–17 sea­son.

▴ Fig­ure 1. In­ci­dence of seed-and soil-borne Rhi­zoc­to­nia stem canker symp­toms at full canopy, av­er­age of all treat­ments.

▴ Fig­ure 2. Soil-borne Rhi­zoc­to­nia _______________________________________________________________________ stem canker symp­toms (left), seed-borne symp­toms (right).

Ta­ble 2. Treat­ment ef­fect on potato tu­ber

mean mar­ketable yield (t/ha) at Lev­els, South Can­ter­bury, in the 2016–17 sea­son.

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