The Complexity of

Silver Scurf

Gary Secor

North Dakota State University

As the need for quality and appearance increases, blemish becomes important

Blemish traditionally caused by the fungus Helminthosporium solani

Silver scurf

No yield loss

Affects quality of tablestock and processing

No foliar phase; only tubers affected

Appears at harvest, primarily on the stem end

Seed borne

Spreads in storage

Symptoms are silver gold metallic discoloration of the periderm

At harvest

Primarily on the stem end

Moves from seed to progeny tubers

Sporulates on seed

Moves by water or via stolon to new tubers

Also affects processing potatoes causing excessive water loss and subsequent peeling difficulties

Infection occurs in the field but

spreads in storage

0

2000

4000

6000

8000

10000

12000

14000

1 31 61 91 121 151 181

Co

nid

ia

/ D

ay

0

100

200

300

400

500

600

700

1 41 81 121 161

Number of Conidia of Helminthosporium solani Collected in Seed

Storage

J-Seed R-Seed

Rodriguez, et al., 1996

0

5000

10000

15000

20000

25000

1 21 41 61 81 101

Co

nid

ia

/ D

ay

1 21 41 61 81 101 121 141

Number of Conidia of Helminthosporium solani Collected in Storage

Chip Storage Table Stock

Rodriguez, et al., 1996

0

5000

10000

15000

20000

25000

1 13 25 37 49 61 73 85 97

Co

nid

ia

/ D

ay

0

200

400

600

800

1000

1200

1400

1 21 41 61 81 101 121

Number of Conidia of Helminthosporium solani Collected in

Chipping Storage

C-Chip D-Chip

Rodriguez, et al., 1996

50

60

70

80

90

100

28 57 85 98Days

Infe

cti

on

In

de

x

0

10

20

30

40

50

60

70

80

90

100

7 21 35 49 63 77 91 105 119

Days

Infection of Minitubers after Exposure for 1 Week in Chipping

Storage

C-Chip D-Chip

Rodriguez, et al., 1996

0

100

200

300

400

500

Mon Tue Wed Thu Fri Sat Sun

Time (Day of Week)

0

20

40

60

80

100

120

Mon Tue Wed Thu Fri Sat Sun

Time (Day of Week)

Nu

mb

er

of

Co

nid

ia / D

ay

0

20

40

60

80

100

120

140

160

1 3 5 7 9 11 13 15 17 19 21 23

Time (Hour of Day)

Nu

mb

er

of

Co

nid

ia / H

ou

r

0

100

200

300

400

500

600

700

2 4 6 8 10 12 14 16 18 20 22 24

Time (Hour of Day)

Number of Conidia of Helminthosporium solani Collected in Storage

Seed Chipping

Rodriguez, et al., 1996

Seed Chipping

Spores of Helminthosporium solani Collected in a Commercial

Potato Storage

0

500

1000

1500

2000

2500

3000

10 13 16 19 22 25 28 10 13 22 25 28 31 3 6 9 12 15 18 21

Dates of Spore Collection

Co

nid

ia / D

ay

Oct. Nov. Dec. Jan.

Rodriguez, et al., 1996

Silver scurf

A disease of potatoes that has no single

control measure, but relies on the cumulative

effect of many small methods to and control -

and it’s still a problem!

A combination of field and storage

strategies

The short version of strategies is:

Rotation: At least one year, H. solani is not soil borne , but is seed

borne

Sanitation H. solani spores may survive in storage but can be

eliminated by cleaning and disinfecting

Seed free of SS: Almost impossible but effective if you can find it

Maxim MZ seed treatment plus Quadris in furrow at planting

Quick harvest no more than 10 days between vine kill and harvest The longer in the ground the more SS

Reduced RH the first 30 days in storage

90%; the trade off is more pressure bruise

Effect of Relative Humidity on Germination of

H. solani

0

20

40

60

80

100

100 99 98 95 92 88

Relative Humidity (%)

Ge

rmin

ati

on

(%

)

Exp. #1

Exp #2

Salas, Secor and Gudmestad, 1996

Resistant varieties.

We don’t have any varieties with good

resistance – but we do have species with

resistance that the breeders are using

Post harvest fungicides

Most H. solani is resistance to TBZ

Quadris and Maxim appear promising and

registration is being pursued

Chlorine dioxide full time in storage at 25

ppm everytime the humidity runs

• Kills spores that move and spread SS during

storage

Silver scurf incidence and severity of tubers stored 16 weeks with

and without chlorine dioxide and incubated six weeks

0

2

4

6

8

10

12

Lesions / Tuber % Area Affected

% S

ilv

er

Sc

urf

No ClO2 With ClO2

Secor and Gudmestad, 1998

H. solani spores collected during a 16 week period from

commercial potato storages with and without chlorine dioxide

0

50

100

150

200

10/7 10/14 10/21 10/28 11/4 11/11 11/18 11/25 12/2 12/9 12/16 12/23 12/30 1/6 1/13 1/20

Time (Week)

Nu

mb

er

of

Sp

ore

s C

olle

cte

d

With ClO2 (15-20 ppm) (F1)

Without ClO2 (FTM)

Secor, Gudmestad and Rivera 1998

Ship

FTM

Ship

FTM

Pump

Broken

Ship

F1

Ship

F1 Ship

F1

Ship

F1 Ship

F1

Ship

FTM Ship

FTM

Colder storage has less sporulation and

SS spread

Effect of Temperature on Germination of

H. solani

0

20

40

60

80

100

20 15 10 5

Temperature (C)

Ge

rmin

ati

on

(%

)

Exp. #1

Exp #2

Salas, Secor and Gudmestad, 1996

Alternative Control

H. solani in vitro doesn’t perform properly all the time

Cultures universally shows black and white growth

Examination of cultures reveals consistent association with contaminating fungus –Acremonium strictum

Research showed A. strictum to be a microparasite affecting H. solani growth

Photos of Helminthosporium cultures with and without Acremonium strictum. A. colony free of A.

strictum. B,C D. Colonies with A. strictum showing sectors and white growth characteristic of

contaminated H. solani cultures.

Scanning electron micrograph of A. strictum; arrow denotes conidia (magnification

500X).

Scanning electron micrograph of H.

solani colonies with (A) and without (B)

Acremonium strictum.

Effects of A. strictum on H.

solani

Reduces

Spore production (sporulation) 35-65 %

Spore germination 43-53 %

Fungal growth 32-40 %

Reduces spore production on minitubers

exposed to H. solani but not on

previously infected tubers

Over time, these strategies appear to be working;

we find less silver scurf

However blemish is still a huge problem –

everywhere; what’s happening??

One of the problems are other causes of blemish

Black dot and physiological

In recent years we have seen an increase in blemish

caused by black dot (Colletotrichum coccodes

Reported to cause “bumpy tuber” in

Pacific NW

Many/most of the controls strategies for silver scurf do not work for black dot, so an accurate diagnosis of the cause of blemish is essential

Foliar black dot can be managed by most fungicides

We do not know the role of seed borne black dot or how to control it

We do not have a control for blemish caused by black dot

Black dot does not spread in storage

Maybes

Seed treatment Maxim ST + Quadris (not Amistar) in furrow; probably/maybe

Quadris post harvest being tested; mixed results

Acremonium bio-control; experimental

PCR soil assay for disease potential (Gudmestad)

Vapam does not work

Surya’s Survey

Tuber blemish survey 2007

Conducted May 2007

Seed and tablestock reds from storage

57 samples

15 tubers per sample incubated and

examined for H. solani and C. coccodes

Very little BD observed

Incidence of SS

Incidence (%) SS # of samples

< 25 9

25-50 9

50-75 8

> 75 31

Severity of SS range 0 to 90%

Conclusion

Even though silver scurf appears to be a

simple disease, control still remains a

challenge due to complexity of the disease