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© 2008 Plant Management Network.
Accepted for publication 17 August 2008. Published 17 November 2008.


Does N or K Nutrition Affect Bronze Wilt in Field-Grown Cotton?


C. O. Gwathmey, Associate Professor, Department of Plant Sciences, M. A. Newman, Professor, Department of Entomology and Plant Pathology, and C. H. Canaday, Associate Professor, Department of Entomology and Plant Pathology, University of Tennessee, 605 Airways Boulevard, Jackson, TN 38301


Corresponding author: Owen Gwathmey. cogwathmey@utk.edu


Gwathmey, C. O., Newman, M. A., and Canaday, C. H. 2008. Does N or K nutrition affect bronze wilt in field-grown cotton? Online. Plant Health Progress doi:10.1094/PHP-2008-1117-01-RS.


Abstract

Bronze wilt (BW) is a disorder of cotton (Gossypium hirsutum L.) that has reduced yields of susceptible cultivars, and may recur in future cultivar releases. Because BW may impair uptake or translocation of mineral nutrients, soil fertility may affect BW incidence or its impact on growth and development. A 3-year study was conducted to determine effects of N and K fertility and tillage on BW incidence and impact on growth and development of field-grown cotton. Incidence of BW was relatively low and unaffected by N or K fertility in this study with or without tillage. Neither N fertility nor tillage affected growth and development responses to BW, although high N fertility delayed appearance of secondary symptoms by an average of 3.5 days. Plant growth response to K fertility was suppressed by BW. Boll retention was reduced more than vegetative growth by BW, but this response was not mitigated by N or K fertilization. Leaves of BW plants had equivalent K concentration as normal plants, but 30% lower P, indicating that BW impaired P uptake or translocation. Results suggest that N and K fertilization are not useful methods to manage BW, but P nutrition merits further research.


 

Fig. 1. Bronze wilt symptoms including leaf bronzing and wilting, reddening of upper canopy, and abnormal shedding of fruit.

What Is Bronze Wilt?

Bronze wilt (BW) is a newly recognized disorder of cotton, generally characterized by a bronze discoloration and wilting of leaves (7) (Fig. 1). Its etiology remains unknown (17,19), although Agrobacterium has been implicated (3,5). Symptoms typically appear in the upper canopy during boll development, when developing fruit may shed abnormally (7,16). Leaves of BW plants are typically warmer than non-affected plants, and stems often turn red as symptoms progress. Symptoms may also include discoloration and necrosis of phloem (7,13), but not the discoloration of xylem associated with Fusarium and Verticillium wilts (7). Bronze wilt was also associated with necrosis of secondary roots in controlled environment studies (4).

Bronze wilt has occurred sporadically since 1995 in susceptible cultivars grown in mid-South and Southeast regions of US cotton production (7). Limited research was conducted on BW symptomatology (7), etiology (5), epidemiology (17), as well as possible management factors (10). Some cultivars were found to be more susceptible than others (4,7), and some product guides to cultivars warn of BW susceptibility at levels which impact yields [e.g., (9)]. Substantial yield losses have been attributed to BW (5,8). Reports of BW injury to US cotton have diminished in recent years, partly due to elimination of susceptible cultivars by commercial seed companies. However, recessive genes conferring susceptibility are carried in several common breeding lines (6), so there is some risk of BW recurrence with future release of new cultivars.


Nutrition Connection

BW is associated with damage to secondary roots, possibly by Agrobacterium spp. (3,5). Damage to feeder roots appears to interfere with nutrient uptake, especially of anionic nutrients such as phosphorus (3). Thus, soil nutrient availability may affect appearance of BW symptoms or effects on plant growth and development. Working in growth chambers, Bell (5) found that BW-induced boll abscission was greatest when phosphorus and potassium were only marginally available. Bell et al. (7) also reported that P and K deficiency favored BW under field conditions. Symptoms of BW have resembled and sometimes been confused with K deficiency (8,16). In some regions where nutrient deficiencies were associated with BW, it was recommended that P or K be used to correct the problem (14). Excess N fertilizer has also been implicated (4). Biomass comparisons in growth chamber studies indicated that BW severity was directly proportional to N fertilizer (6). To lessen the severity of BW, Bell et al. (7) and Albers and Guthrie (2) also advised against over-application of N fertilizer relative to other mineral nutrients. However, Padgett et al. (16) reported no effect of N fertilization on BW incidence in Louisiana field studies.


Research Objectives

One objective of this study was to determine if N or K fertility altered BW incidence and its impact on growth and development in field-grown cotton, with or without tillage to incorporate the fertilizers. A second objective was to determine if plant tissue concentrations of P and K differed between BW and asymptomatic plants as K nutrition varied.


Study Site

This study was superimposed on plots used for long-term soil fertility research at Ames Plantation, Grand Junction, TN. The soil was a non-irrigated Loring-Henry silt loam (fine-silty, mixed, active, thermic Oxyaquic Fragiudalfs, and coarse-silty, mixed, active, thermic Typic Fragiaqualfs). Individual 400-ft² plots had a history of tillage regime and N and K fertilization prior to this study. One reason for selecting this site was its low P fertility, which Bell (3) had suggested as a factor that increased BW severity. Between each year of this study, soil samples were collected in each plot to determine P and K concentrations as described by Howard et al. (12).


Fertility and Tillage Treatments

Two overlapping sets of treatments consisted of factorial RCB arrangements of tillage regime and soil fertility. Plots were managed in one combined field operation but the data were analyzed as separate experiments. Treatments in Experiment 1 were tillage regime, N fertilization (80 and 160 lb/acre) and K fertilization (K2O, 60 and 120 lb/acre). Treatments in Experiment 2 were tillage regime and K fertilization (K2O, 0, 60, 120, and 240 lb/acre), all with N fertilization at 80 lb/acre. Each treatment combination was replicated four times each year. Commercial NH4NO3 and KCl fertilizers were hand broadcast over designated plots each year prior to tillage treatment. Disc and harrow operations incorporated fertilizer in disc-till plots prior to planting, whereas no-till plots were planted without incorporating fertilizer. No P fertilizer was applied during the study.


Planting and Monitoring

Commercial seed of a susceptible cultivar, PM1218BG/RR, was mechanically planted in 40-inch rows on 8 May 2000, 14 May 2001, and 16 May 2002. Except for fertility treatment, the crop was managed according to Agricultural Extension Service (1) recommendations. Plots were monitored weekly for expression of BW symptoms, which first appeared in a few plants at early bloom each year. Two stages of symptom expression described by Bell et al. (7) were observed. Each week, plants showing secondary symptoms (reddening of upper canopy and abnormal shedding of fruit) were counted and flagged until symptoms could no longer be distinguished from normal senescence. Plants showing only primary symptoms (leaf bronzing and wilting only) were not counted or flagged due to the transient nature of those symptoms. Plants showing secondary symptoms were flagged from 60 to 85 DAP (days after planting) in 2000, 67 to 95 DAP in 2001, and 57 to 95 DAP in 2002, for evaluation of impact on plant growth and development.


Tissue Sampling and Analysis

Leaf blades were collected for analysis of tissue P and K concentrations at 74 and 85 DAP in 2000, and 81 and 93 DAP in 2001. The highest fully expanded mainstem leaves were harvested and prepared for analysis as described by Howard et al. (11). Plants showing secondary BW symptoms were harvested separately from asymptomatic plants. Ten samples per plot were harvested from asymptomatic plants on each date, whereas the number of plants with BW symptoms determined numbers of BW samples. Due to low incidence, BW samples were combined across tillage and N treatments to provide sufficient plant material for tissue P and K analysis. Tissue P concentrations were determined by a colorimetric procedure (12), whereas K concentrations were determined by atomic absorption spectrometry (11).


Data Collection and Analysis

Before harvest each year, pairs of flagged (BW) plants and adjacent asymptomatic plants were mapped for height, fruiting branch number, and first position boll retention. These data were collected at 110 DAP in 2000, 119 DAP in 2001, and 102 DAP in 2002. Interactions between treatments and BW were detected by considering pairs of mapped plants to be sub-treatments in the analyses of variance.

All data were first analyzed with a general fixed effects model (Proc GLM) to detect year-by-treatment interactions. In cases where these interactions were smaller than the main effects of treatment, data were analyzed with a mixed model (Proc Mixed) in which years were assumed to have random effects.


Soil Testing

Annual K fertilizer treatments produced stepwise levels of available K fertility ranging from low to very high (Fig. 2). According to Agricultural Extension Service recommendations (1), plots receiving K2O at 0 or 60 lb/acre were under-fertilized with K, while plots receiving K2O at 120 or 240 lb/acre received more K than recommended. Extractable soil P was low to medium, and did not vary with K or tillage treatment. N fertilization rates of 80 and 160 lb/acre were one and two times the recommended rate for cotton, respectively.


 

Fig. 2. Soil test P and K levels in long-term K fertility plots at Ames Plantation, 2000-2002. Letters indicate ranges of low (L), medium (M), high (H), and very high (VH) fertility for cotton, according to criteria of Agricultural Extension Service (1).

 

Incidence of Bronze Wilt

Incidence of BW and plant responses varied with year, but year-by-treatment interactions were small relative to main effects. Therefore, treatment effects were evaluated with a mixed model with years assumed to have random effects.

Incidence of BW varied with year (P < 0.0001), averaging 14.4, 3.4, and 1.4 plants expressing secondary symptoms per thousand plants in 2000, 2001, and 2002, respectively. Incidence in these studies averaged about 10% of those reported by Padgett et al. (17) in Louisiana. Plants with BW symptoms appeared to be randomly distributed within our plots, similar to Padgett et al. A few plants with primary symptoms reverted to an asymptomatic condition each year, but no stem necrosis was observed. The higher incidence of BW in 2000 may be partly attributable to lower rainfall and higher maximum daily temperatures than in the other years. Controlled environment research of Bell (4) indicated that root necrosis and growth reduction associated with BW were more prevalent above 91°F than at lower temperatures.

Tillage regime, N and K treatments did not alter total incidence of BW (Table 1). Across years, however, the higher N rate delayed appearance of BW symptoms by about 3.5 days (P = 0.010). Neither tillage nor K altered the time at which BW symptoms appeared, and did not alter the N effect on symptom appearance (data not shown).


Table 1. Observed significance levels in mixed analysis of variance of bronze wilt (BW) incidence and days to symptom appearance, as affected by tillage regime and mineral nutrition in two experiments at Ames Plantation, TN. Significant (P < 0.05) effects are in bold italics.

Source of variance BW incidencew Days to symptom appearance
Exp. 1x Exp. 2y Exp. 1 Exp. 2
P > F
Tillagez 0.925 0.497 0.675 0.922
N fertilizer 0.500 — 0.010 —
K fertilizer 0.847 0.999 0.115 0.425
Tillage * N fertilizer 0.881 — 0.092 —
Tillage * K fertilizer 0.854 0.216 0.968 0.196
N * K fertilizer 0.639 — 0.873 —
Tillage * N fertilizer * K fertilizer 0.984 — 0.134 —

 w Incidence data subjected to square root transformation.

 x Experiment 1: N rates 80 and 160 lb/acre; K2O rates 60 and 120 lb/acre.

 y Experiment 2: K2O rates 0, 60, 120, and 240 lb/acre.

 z Disc tillage and no-tillage regimes.


Plant Responses to Bronze Wilt

Plant height and first-position boll retention were significantly reduced by BW, but the number of fruiting branches was not affected (Table 2). Across fertility and tillage treatments, BW reduced final plant height 1.2 inches on average in Experiment 1, and 1.7 inches in Experiment 2 (Table 3). Neither tillage nor N altered plant response to BW, but response to K was modified by BW (Table 2). In Experiment 1, BW reduced boll retention by 37 percentage points at the 60-lb K rate, and nearly 50 percentage points at the 120-lb K rate (Table 3). Similar reductions were observed in Experiment 2. BW did not reduce the number of fruiting branches across K rates, but there were minor interactions at the 0 and 120-lb K rates (Table 3). Results indicated that BW reduced boll retention more than it reduced vegetative growth, but these responses were not mitigated by N or K fertilization. Yan et al. (19) also found BW reduced plant height and boll retention, but not mainstem node number, relative to healthy plants. Unlike the findings of Bell (4,6) in controlled environments, we found that plant responses to BW did not vary with N under field conditions. Our N responses are consistent with Padgett et al. (16), who found that N rates did not influence BW in two field studies in Louisiana.


Table 2. Observed significance levels in mixed model analysis of variance of bronze wilt (BW) effects on plant height, fruiting branch number, and boll retention, as affected by tillage regime and mineral nutrition in two experiments at Ames Plantation, TN. Significant (P < 0.05) effects are in bold italics.

Source of variance Plant height Fruiting branches Boll retentionv
Exp. 1w Exp. 2x Exp. 1 Exp. 2 Exp. 1 Exp. 2
P > F
Tillage (Till)y <.0001    <.0001    0.341 0.536 0.405  0.423
N fertilizer (N) 0.511 — 0.394 — 0.461 —
K fertilizer (K) <.0001    <.0001    0.012 0.001 0.431  0.600
Bronze wilt (BW)z 0.024 0.001 0.562 0.813 <.0001 <.0001
Till * N 0.800 — 0.884 — 0.553 —
Till * K 0.503 0.013 0.896 0.390 0.188  0.024
N * K 0.791 — 0.643 — 0.370 —
Till * BW 0.859 0.453 0.880 0.569 0.473  0.630
N * BW 0.338 — 0.195 — 0.423 —
K * BW 0.340 0.963 0.013 0.006 0.015  0.301
Till * N * K 0.608 — 0.864 — 0.003 —
Till * N * BW 0.358 — 0.981 — 0.058 —
Till * K * BW 0.356 0.918 0.102 0.413 0.996  0.908
N * K * BW 0.795 — 0.908 — 0.582 —

 v Retention data subjected to arcsine square root transformation.

 w Experiment 1: N rates: 80 and 160 lb/acre; K2O rates: 60 and 120 lb/acre.

 x Experiment 2: K2O rates: 0, 60, 120, and 240 lb/acre.

 y Disc till and no-till regimes.

 z Bronze wilt symptom expression.



Table 3. Effects of K fertilization and bronze wilt (BW) symptom expression on plant height, fruiting branch number and boll retention across years, tillage treatments and N rates in two experiments at Ames Plantation, TN.

Exp. K fertilizer
(K2O lb/acre)
BW
expression
v
Plant
height

(inches)
Fruiting
branches

(number)
Boll
retention
w
(%)
means across K rates
Exp. 1x — A     28.0 ay      9.4 a     53.5 a
— BW     26.8 b      9.3 a     10.4 b
Exp. 2z — A     28.5 a      9.7 a     53.9 a
— BW     26.8 b      9.8 a     11.8 b
interaction means
Exp. 1 60          A     26.6 bc      9.0 b     51.0 a
60          BW     25.9 c      9.3 b     14.0 b
120          A     29.3 a      9.8 a     56.1 a
120          BW     27.6  b      9.3 b       7.2 c
 
Exp. 2 0          A     26.0 cd      9.6 bcd     50.6 a
0          BW     24.4 d    10.6 a     11.6 bc
60          A     27.7 bc      9.2 cd     51.9 a
60          BW     26.3 cd      9.3 cd     11.6 bc
120          A     29.8 a      9.9 bc     58.3 a
120          BW     27.7 bc      9.2 d       8.0 c
240          A     30.6 a    10.3 ab     54.9 a
240          BW     28.8 ab    10.0 ab     16.6 b

 v A = asymptomatic plants; BW = plants with secondary bronze wilt symptoms.

 w Retention data transformed to arcsine square root for analysis; means reported in original units.

 x Experiment 1: Data were combined across N rates (80 and 160 lb/acre) and tillage since neither had a significant effect on the variables measured.

 y Letters separate means within groups at P = 0.05 by independent pairwise comparisons (pdiff) in Proc Mixed.

 z Experiment 2: N rate was 80 lb/acre. Data were combined across tillage treatments since differences were not significant.


Tissue Response to Bronze Wilt

Leaf P concentrations were significantly reduced by BW relative to normal plants, and these differences were larger at 85 to 93 DAP than at 74 to 81 DAP, due to an increase in leaf P in asymptomatic plants with time (Table 4). All leaf P concentrations were below the critical level of 0.15% cited by Mitchell and Baker (15), but leaf P in most BW plants ranged from 0.06 to 0.08%, indicating severe deficiency. Leaf P concentrations in asymptomatic plants were similar to those of Howard et al. (12) fertilized with no P, but leaf P in BW plants was 30% lower. Given the low boll load on BW plants (Table 3), very low leaf P is more likely due to reduced uptake or transport of P than by reduced P export to developing fruit. Results are consistent with the observation of Bell et al. (3) that BW interferes with uptake of phosphate and other anionic nutrients. In light of observations of phloem necrosis in BW plants (7,13), it is also possible that phloem transport of P is impaired by BW.


Table 4. Effects of sample date, K fertilization and bronze wilt (BW) symptom expression on leaf P and K concentrations across years, tillage treatments and N rates at Ames Plantation, TN.

Sample
date
K fertilizer
(K2O lb/acre)
BW
expression
x
Leaf P
(ppm)
Leaf K
(%)
means across sample dates and K rates
— — A 1066 ay        0.72 a
— — BW 739 b        0.69 a
means across K rates
 1z — A   952 b        0.74 a
1 — BW   732 c        0.75 a
2 — A 1179 a        0.69 ab
2 — BW   746 c        0.63 b
means across sample dates
—    0 A 1087 a        0.46 cd
—    0 BW    777 bc        0.35 d
—   60 A 1048 a        0.57 c
—   60 BW   790 b        0.50 cd
— 120 A 1037 a        0.74 b
— 120 BW   638 c        0.72 b
— 240 A 1090 a        1.10 a
— 240 BW    752 bc        1.18 a

 x A = asymptomatic plants; BW = plants with secondary bronze wilt symptoms.

 y Letters separate means within groups at P = 0.05 by independent pairwise comparisons (pdiff) in Proc Mixed.

 z Date 1 = 74 and 81 DAP in 2000 and 2001 respectively. Date 2 = 85 and 93 DAP in 2000 and 2001 respectively.


Surprisingly, leaf K concentrations were unaffected by BW at either sample date (Table 4). As expected, tissue K concentrations varied with K fertility and decreased with time, but these responses were not altered by BW. The lowest critical level for leaf K at late bloom was cited as 0.75% by Mitchell and Baker (16). Leaf K concentrations were above this level with 240 lb/acre K2O, and near this level with 120 lb/acre K2O, with or without BW. Given the reduced boll load of BW plants (Table 3), we speculate that the reason BW had no effect on leaf K was lower K export to bolls being offset by lower K import from roots of BW plants.


Conclusions

High N fertility delayed the appearance of symptoms by 3.5 days, but incidence of BW and plant growth and development responses were otherwise unaffected by N or K fertility in this study with or without tillage. Results suggest that N and K fertilization are not useful methods to manage BW. Leaves of BW plants had equivalent K concentration as normal plants but 30% lower P, suggesting that BW impaired P uptake and/or translocation. This finding suggests the need to investigate whether P nutrition affects BW in the field.


Acknowledgments

This research was supported in part by the Foundation for Agronomic Research, Cotton Incorporated, and the Hobart Ames Foundation.


Literature Cited

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