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Effects of Crop Rotation, Potato Seed Piece Treatments, and Foliar Fungicide Sprays – They All Contributed to the Final Story in 2002

T. A. Zitter and D. E. Halseth

Departments of Plant Pathology and Horticulture
Cornell University
Ithaca, NY 14853

(Issued April 21, 2003)

Chapter 1 – Seed Piece and In-furrow Treatments, They Really Do Make A Difference
Chapter 2 – Foliar Fungicides Offer Choices and Precautions
Chapter 3 – Crop Rotation, the Benefits You Can’t Afford to Miss
Conclusions – A Story With A Cautionary Good Ending


During the 2002 growing season, we studied the effects of seed piece or in-furrow application of fungicides intended for control of Rhizoctonia black scurf ( Rhizoctonia solani), silver scurf (Helminthosporium solani), and black dot (Colletrotrichum coccodes), and the use of foliar fungicides to control early blight (Alternaria solani) and black dot (C. coccodes). Three distinct varieties were included: the processing variety Andover, and the two fresh market varieties Chieftain and Eva. Serendipity played a role in observing the effects of black dot on canopy health, when part of our Andover plots were rotated 2 years out of potato rather than the usual 1 year at Freeville, NY. The final results were summarized after a final grading and disease assessment of the tubers were made following 6 months in storage. Disease ratings on foliage and stems during the season and tuber assessments at grading all contributed their individual chapters to complete the final story.

Chapter 1 – Seed Piece and In-furrow Treatments, They Really Do Make A Difference

Until recently seed piece and in-furrow treatments were just considered as an added expense that only a few growers would consider using. However, the need to maximize stands and improve tuber appearance has made the use of such treatments a more routine procedure. Rhizoctonia, sprout burn and black scurf, silver scurf, and black dot have become a major issue for stand establishment and daughter tuber appearance.

Background
In 2002 we again paired up seed pieces receiving no fungicide treatments with those receiving a variety of treatments with different seed piece and/or in-furrow treatments, and then followed with a foliar fungicide program. Each treatment was randomized and replicated four times. A separate plot number was assigned to each two-row treatment plot, and this number was used to monitor treatments throughout storage and final disease assessment. Approximately 50 tubers were saved from each treatment in open plastic tubs and stored at 40 F. Tubers of Andover, Chieftain, and Eva were assessed for tuber appearance on four occasions (0 days, 1 mo., 4 mo., and 6 mo. after harvest) using a 0 to 5 scale where 0 = clean and 5 = very poor and unmarketable. The ratings were assigned by consensus of three individuals looking at all treatments within a replication. At the 6 mo. appearance rating, a sub set of 100 tubers were selected at random and assessed in the laboratory for the % surface area of each tuber affected by Rhizoctonia black scurf, silver scurf and black dot.

Results for Andover, Chieftain, and Eva
The seed piece and in-furrow treatments, along with the foliar sprays applied to Andover, Chieftain, and Eva, appear in Tables 1, 2, and 3, respectively. In the case of Andover, treatments 1 and 2 received no seed piece treatments, resulting in tubers with significant infection when compared to seed piece treatments used in treatments 3, 4 and 5. The overall rating for treatments 1 and 2 was consistently poor, displaying significant amounts of silver scurf and black scurf, and higher numerical levels of black dot (Figs 1, 2, and 6). Use of Maxim MZ, Quadris in-furrow, and Maxim MZ + Quadris, (treatments 5, 4 and 3, respectively) performed very well (ranking good, better, and best), and afforded the most daughter tuber protection. Additional foliar sprays of Bravo and Headline did not appear to provide too much added protection for the very susceptible variety Andover, as discussed later.

Table 1.    Effect of seed piece and foliar fungicides on Andover appearance in 2002 after 6 months storage.

Seed piece/in-furrow treatments

 

Foliar treatments

 

Final tuber rating

 

% Black scurf (Rhizoctonia)

 

%
Silver scurf

 

%
Black dot

                     

1. No seed treatment

 

Bravo (6x)

 

4.4 a

 

2.5 b

 

23.5 b

 

7.0

2. No seed treatment

 

Headline (3x)

alt. Bravo (3x)

 

3.9 ab

 

2.7 b

 

24.5 b

 

6.8

3. Maxim MZ ½ lb then Quadris in-furrow 0.958 fl. oz/1000 ft. row

 

Headline (3x)

alt. Bravo (3x)

 

2.5 c

 

0.0 a

 

0.9 a

 

1.0

4. Quadris in-furrow 0.958 fl. oz/ 1000 ft row

 

Headline (3x)

alt. Bravo (3x)

 

2.8 bc

 

0.0 a

 

3.0 a

 

4.5

5. Maxim MZ ½ lb

 

Headline (3x)

alt. Bravo (3x)

 

3.3 abc

 

0.0 a

 

4.0 a

 

3.6

Numbers separated by letters indicate significance at P=0.05

Final tuber rating was assessed on 3/28/2003; three previous ratings gave the same order of performance. Scale 0=clean, 5=very poor and unmarketable

Percent occurrence of Rhizoctonia black scurf, silver scurf and black dot based on surface area infected for 100 tubers examined.


Figure 1 Both rows display the range of symptoms on Andover, with and without fungicide seed treatment (Treatments 1,2,3,4 and 5, left to right). Treatments 1 and 2 (first two tubers in both rows) received no seed piece treatments and show considerable infection by Rhizoctonia (black scurf), silver scurf and black dot. Treatments 3,4 and 5 (middle to right in both rows) were rated as more clean, but occasionally showed escapes, with most infection with the black dot fungus, especially in the upper row (see Table 1). The stolon (stem) end is at the top of each tuber. Figure 2 The normally slightly textured skin of Andover is overwhelmed by a complex of surface-infecting fungi. Upper left, Rhizoctonia black scurf with one patch of silver scurf, but mostly black dot. Upper right, Black dot over the entire tuber surface. Lower left, a silver scurf patch in the middle but surrounded by black dot. Lower right, a patch of Rhizoctonia but with heavy black dot in background. Stolon (stem) end is on the right side for all tubers. Tubers were stored at 40F for 6 months.

The effect of seed piece and foliar fungicides on the appearance of Chieftain is shown in Table 2. In the absence of seed piece or foliar fungicides, the control (Trt. 1) had the worst tuber appearance, showing significant amounts of silver scurf and black dot, and numerically higher levels of Rhizoctonia black scurf. Once again the use of either Maxim MZ with Quadris in-furrow (Trt. 4) or Quadris in-furrow alone (Trt. 5) contributed to significantly better tuber appearance as a result of excellent control of Rhizoctonia, silver scurf and black dot. We did find delayed emergence and reduced yields of Chieftain when Quadris in-furrow was applied at the tested rate. Moncut used in treatment 8, with the active ingredient flutolanil, is known to be most effective against Rhizoctonia, although in this test Rhizoctonia was found on nearly 7 percent of the tubers. Moncut does not have any activity against silver scurf and black dot, but in this treatment the levels of these two pathogens are not significantly different from Treatments 4 and 5. Apparently the use of the foliar fungicides Headline alternated with Endura contributed to reduced occurrence of both silver scurf and black dot.

Table 2.    Effect of selected seed piece and foliar fungicides on Chieftain appearance in 2002 after 6-month storage.

Seed piece/in-furrow treatments

 

Foliar treatments

 

Final
tuber rating

 

% Black scurf (Rhizoctonia)

 

%
Silver scurf

 

%
Black dot

1. No seed treatment

 

None

 

4.5 a

 

4.7

 

27.5 b

 

22 b

4. Maxim MZ ¼ lb. then Quadris in-furrow 0.958 fl. oz./1000 ft. row

 

Quadris (3x)

alt. Bravo (3x)

 

3.1 bc

 

0.0

 

1.2 a

 

0.9 a

5. Quadris in-furrow 0.958 fl. oz./1000 ft. row

 

Headline (3x)

alt. Bravo (3x)

 

3.5 ab

 

0.6

 

4.9 a

 

4.1 a

8. Moncut in-furrow 1.18 fl. oz./1000 ft. row

 

Headline (3x)

alt. Endura (3x)

 

3.0 bc

 

6.7

 

6.4 a

 

6.9 a

Numbers separated by letters indicate significance at P=0.05.

The effect of seed piece and foliar fungicides on the appearance of Eva is shown in Table 3. As was the case with Chieftain, the use of Maxim MZ with Quadris in-furrow or Quadris in-furrow alone, contributed to significant control of silver scurf and black dot (Fig. 3). In this test, the use of Moncut in-furrow performed as well on Eva as did the other fungicide seed treatments for Rhizoctonia control. Once again a reduction in the amount of both silver scurf and black dot was noted when the foliar application of Headline alternated with Endura was used, indicating an apparent benefit from foliar application of these fungicides.

Table 3. Effect of selected seed piece and foliar fungicides on Eva appearance in 2002 after 6-month storage.

Seed piece/in-furrow treatments

 

Foliar treatments

 

Final
tuber rating

 

% Black scurf (Rhizoctonia)

 

%
Silver scurf

 

%
Black dot

1. No seed treatment

 

None

 

4.0 a

 

3.3

 

8.2 b

 

27.3 c

4. Maxim MZ ¼ lb. then Quadris in-furrow 0.958 fl. oz./1000 ft. row

 

Quadris (3x)

alt. Bravo (3x)

 

2.9 bc

 

1.1

 

1.9 a

 

1.9 a

5. Quadris in-furrow 0.958 fl. oz./1000 ft. row

 

Headline (3x)

alt. Bravo (3x)

 

3.5 ab

 

0.3

 

1.6 a

 

1.3 a

8. Moncut in-furrow 1.18 fl. oz./1000 ft. row

 

Headline (3x)

alt. Endura (3x)

 

3.0 bc

 

0.6

 

4.6 ab

 

10.6 b

 

Numbers separated by letters indicate significance at P=0.05. Final tuber ratings made 3/28/2003; three previous ratings gave similar results; scale 0=clean, 5=very poor and unmarketable. Percent occurrence of Rhizoctonia black scurf, silver scurf and black dot based on surface area infected for 100 tubers examined; Endura (nicobifen) is unregistered, was tested as BAS510.

 
Figure 3 The normally clean and bright appearance of Eva is shown on the left. Next to it are tubers exhibiting Rhizoctonia black scurf, silver scurf, and a black dot infected tuber on the far right. The picture was taken 6 months after storage at 40F, although the differences among the treatments used were apparent at the time of grading in October 2002.

Chapter 2 – Foliar Fungicides Offer Choices and Precautions

Life used to be so simple. If asked why a grower used foliar fungicides, most would answer ‘because they are needed for control of both early and late blight’, but today that answer would only be partially correct.

Background

Growers in the past relied heavily upon protectant fungicides such as chlorothalonil (Bravo), mancozeb (Dithane), metiram (Polyram), triphenyltin hydroxide (TPTH, Super Tin), and fixed coppers, and these fungicides continue to form the backbone of a preventative program for blight control. However, we now know that repeated use of some of these materials without alternation with different chemistries can result in A. solani and A. alternata becoming less sensitive to chlorothalonil and TPTH, and to azoxystrobin (Quadris) and mancozeb, respectively. With the advent of new genotypes of the late blight fungus, Phytophthora infestans, growers discontinued using Ridomil (metalaxyl) or Ridomil Gold (mefenoxam) and switched to fungicides like Curzate, Tattoo C (now called Previcur Flex, which is not yet labeled in New York), or Gravel (also not currently labeled). With new fungicides utilizing strobilurin chemistry, the release of azoxystrobin (Quadris) in 1999, and new products Gem and Headline, growers now have new fungicides to alternate with protectants. However, a precautionary note needs to be sounded. Because strobilurins have a single-site mode of action (they are called QoI fungicides and inhibit mitochondrial respiration in fungi, as do other classes of fungicides), there is the risk of fungi overcoming this form of control mechanism. This resistance problem has already occurred when using Quadris on cucurbits for the control of powdery mildew in New York and gummy stem blight in several southern states. Less sensitivity with the use of Quadris for A. solani has also been noted in potato in the Midwest, although we have not identified this problem in New York. Clearly the message to rotate strobilurin chemistry is real and applies to all three strobilurin fungicides (labeled [Quadris and Gem] or soon to be labeled in NYS [Headline]). In an effort to prevent further widespread resistance problems from developing, Syngenta will recommend the mixing of Quadris with Bravo in NY, while in other states the combination will be introduced as Quadris/Bravo Performance Pak. Additionally the company is recommending that if Quadris was used as an in-furrow treatment (see Chapter 1), then the first foliar spray should not include a strobilurin. If resistance is present on a farm or an adjacent field, then no strobilurin (any of the three) should be used for the entire season. The other fungicide companies have also stressed the need for a fungicide resistance management program.

Results – Early blight and black dot control
During the past three seasons we have tested the use of strobilurins (mainly Quadris and Headline), principally for early blight control. Efficacy of the products for early blight control was assessed by inoculating the outside rows of all treatments with Alternaria solani, and disease development was recorded weekly, with the progress of the disease recorded as area under the disease progress curve (AUDPC) (Table 4.). Quadris or Headline when alternated with Bravo (or Endura in our 2002 trial) provided significantly improved control of early blight. Both the 2001 and 2002 growing seasons were very stressful for potato production, and by the end of August in both years, treatments not receiving the strobilurins in alternation with chlorothalonil or a few other fungicides like Endura had foliage and vines that were nearly dead (Fig. 4). By examining the stems from each plot we found that black dot (Colletotrichum coccodes) was largely responsible for the early vine decline observed (Fig. 5). Examination of stems receiving Quadris or Headline in alternation with good protectant chemistry showed significantly less black dot (Trts. 4, 5, and 8 in Table 4). The control of black dot on the stems also contributed to a significant reduction in the occurrence of black dot on potato tubers, although it is not always easy to assess when control afforded by a seed piece or in-furrow treatment left off and the foliar fungicide took over. Certainly the foliar fungicides contributed to the healthier vines. A significant increase in specific gravity, shown here for Chieftain and Eva (but also demonstrated with Andover) has also been noted for the last few seasons when strobilurin fungicides were included in the foliar fungicide program.

Table 4. Effect of selected foliar fungicides on potato disease control and tuber qualities for Chieftainand Eva in 2002.

Treatment and rate/A

 

AUDPCz for early blight

 

Black dot severity on stems

 

Tuber appearancey

 

Specific gravity

1. Control, no spray

 

3.35 ax

 

4.3 a

 

3.8 a

 

1.0705 bc

4. Quadris (12.3 fl. oz.) alt. Bravo Ultrex (1.8 lb), 3x each

 

1.04 d

 

1.7 cd

 

1.8 c

 

1.0728 a

5. Headline (9.2 fl. oz.) alt. Bravo Ultrex (1.8 lb), 3x each

 

0.97 d

 

0.97 d

 

1.9 bc

 

1.0728 a

8. Headline (9.2 fl. oz.) alt. Endura (0.14 lb)w, 3x each

 

1.18 cd

 

1.18 cd

 

2.4 bc

 

1.0729 a

z Area under the disease progress curve.

y Tuber appearance graded on a 0-5 scale where 0=clean and excellent and 5=very poor and combined for both varieties.

x Means in columns followed by some letter are not significantly different P=0.05.

w alt=alternating spray program; Endura (BAS 510) nicobifen, is not registered by U.S. EPA.

Figure 4 Early death of the potato canopy for Eva and Chieftain as seen in fungicide trials conducted at Freeville, NY in 2001. Treatment on the left had no fungicide for black dot control, while the treatment on right was sprayed with the strobilurin fungicide Headline. Similar results were observed in 2002. Picture was taken on 9/7/01, and the green weeds are only in the alleyway between the plots. Figure 5 Heavy colonization of potato stems, roots and stolons by the black dot fungus (Colletotrichum coccodes) was common for Chieftain, Eva, and Andover varieties studied during the 2001 and 2002 growing seasons. Note that black dots occur both internally and externally on the stem tissue.

Chapter 3 – Crop Rotation, the Benefits You Can’t Afford to Miss

Growers have been well schooled in the importance of rotations, and in New York most growers can at best work with a 3-year rotation out of potatoes. However, even this can have a major impact on the amount of disease that can occur the following year. Although many potato pathogens are seed borne, and include the organisms we studied this past year (Rhizoctonia, silver scurf, black dot, and early blight), soil borne inoculum is just, if not more important, for black dot, Rhizoctonia and early blight occurrence.

Background
Black dot (C. coccodes ) is named from the abundant small, black sclerotial bodies that form on tubers (barely visible to the naked eye), roots, stolons, and above ground stems (appearing about the size of a pin point) (Figs 5 and 6). The high incidence of black dot on infected potato stems and the ability of sclerotia to survive in New York soils for at least 8 years, means that even in seasons when yield reduction and/or tuber infection may be negligible, the fungus will continue to survive in New York soils in ever increasing numbers. Although black dot is normally considered a low-grade pathogen of potato, its importance as a silent killer of stems and vines is greatly underestimated, and can result in yield losses in excess of 10-20%. Foliar symptoms can be confused with normal plant senescence, and thus blamed on other plant diseases such as early blight (Alternaria solani), Verticillium and Fusarium wilts, and a number of stress factors (potato leafhopper and drought). Black dot can cause tuber skin discoloration (background discoloration or a sooty appearance), which can be confused with the discoloration caused by silver scurf (Helminthosporium solani), so again the presence of the fungus can be underestimated.

Figure 6 Microsclerotia of black dot are seen as black pepper-like specks in a camera close up view of the Andover tuber surface. A hand lens would also help for identification. Surprisingly, the sclerotia are singly spaced, round black bodies that rest on the surface. In contrast, silver scurf colonization without added humidity will appear as compressed black growth that is irregular in shape, and this is associated with a silvery patch.

Figure 7 Graphic appearance of Andover plots as early vein death (yellow color) progressed during the 2002-growing season. Replications 1 and 2 were rotated out of potato for only 1 year, and showed nearly 90% dead vines by the end of August (Aug 26). Plots in area occupied by reps 3 and 4 were out of potato for 2 years, and still had considerable green tissue at the end of August. Eventually even these plots had over 50% dead tissue on Sept 9, 2002.

(Click on image for larger view)

Results
Serendipity occasionally plays a role in field research. The 2002 season was our first opportunity to work with the variety Andover, and afforded us the opportunity to determine if newer fungicides could improve canopy health and thus the yield and quality of this variety. The Andover field plot is shown in Fig. 7, and consisted of a randomized block design with four replications spread from left to right across the field. The most effective seed piece and foliar fungicides were tested as described in Chapter 1. Weekly ratings of the canopy health were made, and by August 26 substantial differences in the appearance of the four replications was apparent (Fig. 7). Since fungicide treatments were the same across all replications, the most plausible explanation for this pattern was the cropping history for this field. Replications 1 and 2 were rotated out of potato for only 1 year, and showed nearly 90% dead vines by Aug. 26. In contrast, plots occupied by replications 3 and 4 had been out of potatoes for 2 years, and remained greener for an additional 3 weeks. Not surprisingly, total yields averaged 28 cwt. higher from treatments in rep 4 compared to rep 1, about a 10% yield increase. Examination of the stems from each replication showed that the stems sampled from reps 1 and 2 averaged 35-65% higher levels of black dot infection. Two points can be made from these results. First, that rotation can play a critical role in reducing the level of soil borne pathogens like Colletotrichum, as this inoculum can remain in the soil for extended periods of time. Secondly, even when the best available chemistries for seed piece and in-furrow treatments along with the best foliar fungicides are used, they alone can not over come a situation of high soil borne inoculum, a stressful and conducive season, and a temperamental variety like Andover.

Conclusions – A Story With A Cautionary Good Ending

Growers now have a range of fungicides for use as seed piece or in-furrow treatments. As shown in Chapter 1, either Quadris in-furrow or Maxim MZ seed treatment provided excellent control of silver scurf, black dot and Rhizoctonia. The best results were obtained when the two fungicides were used together, although the differences were not significantly better than using either product alone.

The positive impact of using strobilurin fungicides like Quadris and Headline has been seen in excellent control of early blight, reduction of early vine decline caused by black dot, greener canopies, higher yields, and higher specific gravities (Chapter 2). We have less experience with Gem (Flint in other vegetables), so our comments have been limited to Quadris and Headline, used in rotation with Bravo. Other protectant fungicides could be substituted for the chlorothalonil sprays, but given that our tests were conducted under high disease pressure for both early blight and black dot, we would have expected some reduction in overall control if other protectants had been substituted. It is critical that resistance management strategies be followed by all growers for the different families of fungicides, especially now for strobilurins. To lose their effectiveness would be a step backward in controlling the complex of seed and soil borne pathogens of potato. Although our experiments were not designed to determine the optimal timing for strobilurin applications, our observations suggest that at least one foliar spray should be applied before the end of July and another in mid-August, in order to maximize the expectant benefits.

Finally, the need to extend the rotational periods between potato crops cannot be overemphasized (Chapter 3). Although most growers are limited to a 2 or 3-year rotational program, the direct benefits of this would be seen immediately in higher yields and better quality tubers.

Acknowledgements
This research was supported in part by the NYS Potato Research and Development Fund grant awarded to T. Zitter and D. Halseth for 2002. We also thank J. Drennan, L. Smith, E. Sandsted, R. MacLaury and J. Kelley for assistance during various phases of this research. We express thanks to Kent Loeffler in Plant Pathology Department for his studio photographic skills.