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Production of huitlacoche, Ustilago maydis: timing inoculation and controlling pollination

     Department of Crop Sciences, University of Illinois, Urbana, Illinois

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 

Huitlacoche is the name given to young, fleshy, edible galls that form when ears of Zea mays are infected by Ustilago maydis. Huitlacoche is processed and sold fresh at markets in Mexico. Interest has increased recently in producing U. maydis as a specialty mushroom in the United States. Silk-channel inoculation methods developed to evaluate common smut resistance in maize can be used to produce huitlacoche commercially. This research assessed the effects of time of inoculation and preventing pollination on the severity of ear galls and yield of huitlacoche produced by inoculating silks with U. maydis. Yield of huitlacoche and severity of ear galls were closely related, as was evident from highly significant linear or curvilinear regressions. Severity and yield were greatest when ears were inoculated 4–8 d after the mid-silk growth stage. Ear galls were 5–15% more severe and yield of huitlacoche was 18–150% greater on ears that were not pollinated, compared to those that were pollinated. Maximum yield of huitlacoche was 131 g ear^-1 from unpollinated male-sterile field corn inoculated 6 d after the mid-silk growth stage and 92 g ear^-1 from detasseled sweet corn inoculated 6 d after mid-silk. About 25% of the total weight of ears consisted of marketable huitlacoche when yields were highest. Quality of huitlacoche was not affected by time of inoculation or pollination treatments, but quality of huitlacoche harvested 12–14 d after inoculation was unacceptable primarily due to lack of teliospores, which affected color and flavor.

Key words: common smut, cuitlacoche, specialty mushrooms, Ustilago zeae

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
Throughout the world, mushroom production increased nearly 20-fold in the past 35 years (Royse 1997). Most of this increase occurred during the past 15 years with a dramatic shift in the genera of fungi that were produced and sold commercially. Agaricus bisporus accounted for about 70% of the world supply of commercial mushrooms in 1979 but only 32% by 1997. A variety of common and exotic mushrooms currently are produced worldwide and in the United States.

Specialty mushrooms are a relatively new commercial enterprise in the U.S. (Royse 1997). During the past 15 years, specialty mushroom production in the U.S. increased an average of 20% per year. In 2000, the value of specialty mushrooms in the U.S. was nearly $45 million, with an average price of all sales near $6.40 Kg^-1 (U.S.D.A. 2000).

Huitlacoche (Syn. cuitlacoche) is the native Mexican name given to young, fleshy, edible galls that form when ears of maize (Zea mays L.) are infected by the basidiomycete Ustilago maydis (DC) Corda (Syn. U. zeae Unger). Throughout the world, these galls are best known as common smut or boil smut, a destructive disease of maize (Smith and White 1988). In central Mexico, huitlacoche is a highly prized delicacy that has been eaten since Precolumbian times (Valverde et al 1995, Vanegas et al 1995). About 400–500 tons of huitlacoche are sold annually during July and August at markets in Mexico City (Villanueva 1997). Huitlacoche also is processed by at least six companies that sell the specialty mushroom canned or lyophilized. Interest in producing U. maydis as a specialty mushroom in the U.S. has increased recently due to an emerging acceptance of huitlacoche by the North American public, who view it as a gourmet food item that is part of a growing market for haute Mexican cuisine (Bayless et al 1996, Kennedy 1989). Huitlacoche can be purchased on the Internet. It is sold by a few specialty growers in the U.S. for as much as $30 to $40 Kg^-1.

Efforts to develop an efficient method of inoculating maize with U. maydis began in the 18th century when Tillet unsuccessfully attempted to demonstrate the causal relationship between common smut and U. maydis. Many inoculation methods were examined in the past century to evaluate smut resistance in maize (Walter 1935, Christensen 1963). Some evaluations of maize resistance employed inoculation methods that induced ear galls (Kerns et al 1999, Pataky et al 1995, Thakur et al 1989). These inoculation methods also can be used to produce huitlacoche (Pataky 1991, 2002; Pope and McCarter 1992; Snetselaar and Mims 1993; Zimmerman and Pataky 1992). A silk-channel inoculation procedure resulted in a significantly higher incidence of ear galls than natural infection, but assessments of hybrid reactions to smut over the years were inconsistent using this method (Pataky et al 1995). Variation associated with this inoculation procedure was due partly to inoculum concentration and partly to differences among people performing the inoculations (du Toit and Pataky 1999b). Pollination and silk age also affected the susceptibility of ears to infection by U. maydis (du Toit and Pataky 1999a). Within 6–24 h after the first pollen tube reaches a maize ovary, an abscission zone of collapsed cells forms from disorganized tissue at the base of the silk (Heslop-Harrison et al 1985). Maize kernels appear to be protected from infection by U. maydis after the formation of the abscission zone because U. maydis infection filaments are unable to grow past this layer of dead cells (Snetselaar et al 2001). About 1 wk after the silks emerge from ear shoots, tissues at the base of the silk begin to collapse due to senescence (Bassetti and Westgate 1993a, b). Senescence of silks before pollination reduces the number of kernels per ear (i.e., successful fertilizations) and it probably prevents U. maydis from colonizing ovaries in a manner similar to the formation of the abscission layer. Efficient production of huitlacoche by inoculating silks with U. maydis consequently may require accurate timing of inoculation and control of pollination to maximize the number kernels infected (i.e., number of infected ovaries) and yield of huitlacoche.

In preliminary studies to assess yield and quality of huitlacoche, sweet corn hybrids were screened as potential plants on which to produce the fungus, and ear-gall development was monitored after silk-channel inoculation (Pataky 1991, Valverde et al 1993). Weight of ears with galls reached a maximum of about 280–600 g ear^-1 14–21 d after inoculation. Gall tissue was nearly 100% black 19–21 d after inoculation. Yield and quality of huitlacoche were optimal during a 1- to 2-d harvest window about 16 d after inoculation when 60–80% of gall tissue was black and weight of ears with galls was approaching the maximum. Soon after this optimal time of harvest, galls lost their spongy integrity and galls that were not protected by husk were colonized intermittently by other microbes (e.g., Fusarium spp., Penicillium spp., bacteria). In addition, gall size, incidence of ears with galls and protection of mature galls by husk differed substantially among 400 sweet corn hybrids evaluated. Based on these and other preliminary studies in Mexico (Villanueva 1997), it appears that huitlacoche can be produced by inoculation, although additional research is needed to further increase production efficiency. The objectives of this research were to assess the effects of timing of inoculation and control of pollination on yield of huitlacoche.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
Fungal isolates and inocula. Inocula were produced for all experiments using methods similar to those described by Snetselaar and Mims (1993). Compatible isolates of U. maydis (i.e., isolate 521 of mating type a1b1 and isolate 518 of mating type a2b2) were maintained separately as sporidia on acidified potato-dextrose agar (aPDA). Isolates were obtained originally from K. M. Snetselaar, St. Joseph's University, Philadelphia, Pennsylvania 19131. Liquid sporidial cultures were prepared in 100 mL Erlenmeyer flasks by seeding 50 mL of sterile potato-dextrose broth (PDB) with sporidia taken from aPDA plates. PDB cultures of U. maydis were incubated at room temperature 18–24 h on a shaker oscillating 120 times per min. To produce inocula, sterile PDB in 2 L Erlenmeyer flasks were seeded with aliquots from liquid cultures at a ratio of approximately 0.5 mL sporidial suspension: 100 mL sterile PDB. The volume of seeded PDB varied depending on the amount of inoculum required for each inoculation. Cultures of inocula were incubated on a shaker 12–18 h. Sporidial isolates were mixed 1:1 (volume:volume) and inoculum was diluted to about 10⁶ sporidia mL^-1 immediately before use.

Sporidial suspensions were injected down the silk channel of ears. Inoculum was delivered with backsprayers (model 425, Solo Inc., Newport News, VA) equipped with an injection device consisting of a Tee-Jet brand meter jet (Model 23623–31, Spraying Systems Co., Wheaton, Illinois) fitted with a 0.64 cm Tee-Jet outlet adapter (Model 4676). A grease needle (Model 5803, Lincoln Automotive, St. Louis, Missouri) with the orifice enlarged to 1.6 mm was attached to the Tee-Jet assembly to inoculate ears in the silk channel. Primary ears of all plants in each experiment were inoculated. If silks had not yet emerged, inoculum was injected through the upper leaves of the developing ear shoot.

Time of inoculation experiment. Time of inoculation and harvest were evaluated in an experiment with a 3 x 6 factorial treatment design. The three inoculation treatments included plants inoculated 2 d after mid-silk (mid-silk = silks beginning to emerge on 50% of plants in a plot), 4 d after mid-silk and both 2 and 4 d after mid-silk. The six harvest treatments included consecutive harvests 12–17 d after inoculation. Treatments were arranged in a split-plot of a randomized complete block design with four replicates. Each experimental unit was a two-row plot with about 35 plants per each 8 m row. The sweet corn hybrid "Sure Gold" was planted 27 Apr 2000. Primary ears of all plants in a plot were inoculated 5, 7, or 5 and 7 Jul using the silk-channel method with 8 mL of inocula injected into each silk channel. Plants inoculated 2 and 4 d after mid-silk were inoculated with 8 mL each time. All primary ears in a plot were harvested by hand and weighed in bulk by experimental unit. Each ear was assessed for percentage of kernels infected (i.e., severity of galls on ears) and placed in one of eight categories: 0%, 1–5%, 6–20%, 21–40%, 41–60%, 61–80%, 81–95% and >95%. Large and small smut galls were assessed similarly so that these ordinal ratings reflected the percentage of kernels (i.e., ovaries) infected. Ears that were rotted by secondary microorganisms were placed a separate category. Galls were removed from ears and marketable huitlacoche was weighed from all ears in a plot. Recovery of huitlacoche was calculated as the weight of marketable huitlacoche removed from ears as a percentage of the total ear weight per plot.

Data were analyzed by analysis of variance (ANOVA) and means were compared by Waller-Duncan Bayesian minimum significant difference values, BLSD, (k = 100) when F-tests indicated significant differences among treatments. Also, a 1 Kg sample of huitlacoche from each treatment was frozen and sent to Tracey Vowell, managing chef of Topolobampo and Frontera Grill restaurants in Chicago for a subjective assessment of cooking and eating quality. Each sample was judged as acceptable or unacceptable for use in a variety of recipes based on color, texture and flavor (Tracey Vowell, pers comm).

Time of inoculation and control of pollination experiments. Two sets of experiments examined the effects of time of inoculation and control of pollination on yield of huitlacoche. The sweet corn hybrid "Jubilee" was planted 11 and 23 Apr and 10 May 2001 for three trials of the first experiment. A 2 x 5 factorial set of treatments were arranged in a split-plot experimental design with three replicates. Two pollination treatments, unpollinated (e.g., detasseled) or pollinated (e.g., not detasseled) were applied to main plots and plants in subplots were inoculated at one of five different times. Pollination was prevented by removing tassels by hand when tassels first emerged from leaf whorls, or tassels were not removed, which allowed plants to pollinate normally. Plants were inoculated once at 2, 4, 6, 8 or 10 d after mid-silk beginning 27 Jun and 3 and 14 Jul for each trial, respectively. Eight mL of a sporidial suspension were injected into silk channels of primary ears as described previously. Each experimental unit was a single row with about 65 plants per 18 m row. A buffer zone of 18 m of detasseled plants separated each replicate of detasseled (unpollinated treatment) and normally pollinated plants in each main plot. All primary ears in a plot were harvested by hand 16 d after inoculation and weighed in bulk by experimental unit. Severity of ear galls was assessed, weight of huitlacoche was measured and recovery of huitlacoche was calculated, as described previously. Categories of percentage of kernels infected (i.e., severity of galls on ears) were slightly different than in the previous experiments: 0%, 1–5%, 6–10%, 11–25%, 26–50%, 51–75%, 75–90% and >90%.

In the second experiment, three male-sterile field corn hybrids (LH202 x LH172, LH198 x LH185 and LH235 x LH185) were planted 9 May 2001 and grown in isolation, where they remained unpollinated (i.e., male-sterile) or they were pollinated by hand. The three hybrids differed in maturity by about 6 d. Each hybrid was a separate trial with two replicates of a 2 x 8 factorial set of treatments arranged in a split-plot design of a randomized complete block. Two pollination treatments (unpollinated or pollinated) were applied to main plots and plants in subplots were inoculated at one of eight times. When silks began to emerge for each hybrid, pollen was collected from a neighboring field of sweet corn, carried to designated plots in paper bags and placed on silks of primary ears of plants in the pollinated treatment. This procedure was repeated three times at 2 d intervals. Plants were inoculated once at 2, 4, 6, 8, 10, 12, 14 or 16 d after mid-silk beginning 14, 18 and 20 Jul for the three hybrids, respectively. Eight mL of a sporidial suspension were injected into silk channels of primary ears, as described previously. Each experimental unit was a two-row plot with about 35 plants per each 8 m row. All primary ears in a plot were harvested by hand 16 d after inoculation and weighed in bulk by experimental unit. Severity of ear galls was assessed, weight of huitlacoche was measured and recovery of huitlacoche was calculated, as described previously.

Data were analyzed by ANOVA as described above. Yield of huitlacoche and severity of ear galls were compared by regression for all experiments. All analyses were done with Statistical Analysis System (SAS) software (SAS Institute, Cary, North Carolina). Quality of frozen huitlacoche was assessed subjectively by T. Vowell, as described previously.

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
Time of inoculation and harvest. The percentage of kernels infected on each ear (i.e., severity of ear galls) differed among inoculation treatments. Severity of ear galls was not affected by time of harvest, and there was no interaction between inoculation treatments and harvest dates. Mean severity of ear galls was 52%, 46% and 55% for plants inoculated 2, 4 and 2 and 4 d after mid-silk, respectively (Table I). Although differences among treatment means were small, distributions of ear-gall severity differed considerably among inoculation treatments (Fig. 1). Plants inoculated 2 d after mid-silk had a large percentage of ears (21%) without galls and a large percentage of ears (28%) with >95% of the kernels infected with U. maydis (Table I, Fig. 1). Only 31% of the plants inoculated 2 d after mid-silk were in the middle of the distribution with 6–60% ear-gall severity. Conversely, plants inoculated 4 d after mid-silk had a smaller percentge of ears (8%) without galls, a large perentage of ears (53%) with 6–60% ear-gall severity, but only about 17% of the ears had >95% severity (Table I, Fig. 1). The distribution of ear-gall severity for plants inoculated 2 and 4 d after mid-silk was similar to the distribution for plants inoculated 2 d after mid-silk, except that fewer ears (6%) escaped infection (i.e., produced no galls) and nearly 7% of the ears were rotted, presumably by bacteria that were favored by an excessive amount of inoculum (Table I, Fig. 1).

View larger version (44K): [in this window] [in a new window]   FIG. 1. Distribution of gall severity (%) when primary ears of the sweet corn hybrid "Sure Gold" were inoculated with U. maydis 2, 4, or 2 and 4 d after the mid-silk growth stage

Time of inoculation and control of pollination. Mean severity of ear galls was affected by time of inoculation, pollination treatments and an interaction between these independent variables (Figs. 2, 3). Ear galls were about 15% more severe on detasseled sweet corn than on pollinated sweet corn, except when plants were inoculated 2 d after mid-silk (Fig. 2). Ear galls were about 5–10% more severe on male-sterile field corn than on pollinated field corn when plants were inoculated 6, 8 or 10 d after mid-silk (Fig. 3). Distributions of ear-gall severity were similar for detasseled and pollinated sweet corn inoculated 2 d after mid-silk, when mean severity of ear galls was 41 or 44%, respectively (Fig. 4). Distributions of ear-gall severity also were similar for unpollinated and pollinated male-sterile field corn inoculated 2 d after mid-silk, when mean severity of ear galls was 35 or 42%, respectively (Fig. 5). When sweet corn was inoculated 8 d after mid-silk, distributions of ear-gall severity differed between detasseled and pollinated (i.e., not detasseled) treatments as fewer detasseled plants escaped infection and a larger percentage of detasseled plants had ears that were at least 25% infected (Fig. 4). When male-sterile field corn was inoculated 8 d after mid-silk, distributions of ear-gall severity differed slightly between unpollinated (i.e., male-sterile) and pollinated treatments as a higher percentage of ears from pollinated plants had more than 50% of the kernels infected by U. maydis (Fig. 5).

View larger version (21K): [in this window] [in a new window]   FIG. 2. Severity of ear galls (%) when the sweet corn hybrid "Jubilee" was detasseled or allowed to pollinate normally and inoculated with U. maydis 2, 4, 6, 8 or 10 d after the mid-silk growth stage

View larger version (20K): [in this window] [in a new window]   FIG. 3. Severity of ear galls (%) when three male-sterile field corn hybrids were grown in isolation (i.e., not pollinated) or pollinated by hand and inoculated with U. maydis 2, 4, 6, 8, 10, 12, 14 or 16 d after the mid-silk growth stage

View larger version (29K): [in this window] [in a new window]   FIG. 4. Distribution of ear-gall severity (%) when the sweet corn hybrid "Jubilee" was detasseled or allowed to pollinate normally and inoculated with U. maydis 2 or 8 d after the mid-silk growth stage

View larger version (23K): [in this window] [in a new window]   FIG. 5. Distribution of ear-gall severity (%) when three male-sterile field corn hybrids were grown in isolation (i.e., not pollinated) or pollinated by hand and inoculated with U. maydis 2 or 8 d after the mid-silk growth stage

View larger version (19K): [in this window] [in a new window]   FIG. 6. Yield of huitlacoche (g ear-1) from a sweet corn hybrid "Jubilee" that was detasseled or allowed to pollinate normally and inoculated with U. maydis 2, 4, 6, 8 or 10 d after the mid-silk growth stage

View larger version (19K): [in this window] [in a new window]   FIG. 7. Yield of huitlacoche (g ear-1) from male-sterile field corn hybrids that were grown in isolation (i.e., not pollinated) or pollinated by hand and inoculated with U. maydis 2, 4, 6, 8, 10, 12, 14 or 16 d after the mid-silk growth stage

The relationship between yield of huitlacoche and severity of ear galls differed between sweet corn and male-sterile field corn. Yield of huitlacoche was linearly related to severity of ear galls on sweet corn (Fig. 8). This relationship was best explained by the regression equation: Y = -5.8 + 1.45X, (r² = 0.86) where Y = yield of huitlacoche (g ear^-1) and X = severity (%) of ear galls. The relationship between huitlacoche yield and ear-gall severity for the male-sterile field corn hybrids was best explained by a curvilinear relationship, Y = 6.6 + 0.03X² (r² = 0.95, Fig. 8). When ear-gall severity ranged from 0 to 50%, yield of huitlacoche was similar from sweet corn and field corn; however, when severity was above 50%, yields of huitlacoche were greater from field corn.

View larger version (23K): [in this window] [in a new window]   FIG. 8. Relationships between yield of huitlacoche (g ear-1) and severity of galls (%) when sweet corn hybrids "Sure Gold" and "Jubilee" or male-sterile field corn hybrids were inoculated with U. maydis using a silk-channel inoculation procedure

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
Ustilago maydis is one of several edible fungi that are part of an expanding market for specialty mushrooms. Based on this research and previous work (Pataky 1991, 2002; Pope and McCarter 1992; Snetselaar et al 2001; Valverde et al 1993; Villenueva 1997; Zimmerman and Pataky 1992), huitlacoche production appears to be maximized when silks of unpollinated, large-eared field corn cultivars are inoculated with sporidia of U. maydis about 5 d after silk emergence. Optimal yield of high quality huitlacoche occurs about 16 d after inoculation. Additional evaluation of corn cultivars, isolates of U. maydis, environmental factors that affect gall maturity and size and research that identifies improved methods of harvest and post-harvest storage further will enhance huitlacoche production systems.

A silk-channel inoculation technique developed to assess reactions of corn to common smut can be used to improve the efficiency of production of huitlacoche. Yield of huitlacoche is closely related to the percentage of kernels (i.e., ovaries) per ear that are infected by U. maydis (i.e., severity of ear galls). Yield of huitlacoche and severity of ear galls are affected substantially by timing of inoculation and by pollination. Yield and quality of huitlacoche also are affected greatly by time of harvest.

Yield and quality were maximized in our first experiment when huitlacoche was harvested about 16 or 17 d after inoculation. Quality was unacceptable when huitlacoche was harvested before 15 d after inoculation. This corroborates previous observations from a preliminary study in which yield and quality of huitlacoche were optimal 17–19 d after inoculation (Valverde et al 1993). Optimal harvest time probably is affected slightly (e.g., 2 or 3 d) by growing degree d. The possibility that variation among U. maydis isolates might affect yield and quality of huitlacoche also needs to be examined further.

Severity of ear galls and yield of huitlacoche were greatest when ears were inoculated 4–8 d after the mid-silk growth stage. During this time, silks have emerged from ear shoots of most plants, but the most mature silks have not yet begun to senesce. When plants were inoculated 2 d after mid-silk, a large percentage of ears escaped infection, presumably because silks had not yet emerged from some plants for which maturity was delayed a few d. When plants were inoculated later than 8 d after inoculation, severity and yield were low, presumably because silks began to senesce (Bassettii and Westgate 1993a, b) and U. maydis infection filaments were unable to reach the kernels. Previously, du Toit and Pataky (1999a) observed that the incidence of ears with galls differed by as much as 20% when plants were inoculated 3 d apart and by as much as 70% when inoculation was delayed 1 wk. This decline in susceptibility also was attributed to silk senescence.

Severity of ear galls and yield of huitlacoche also were affected by pollination. Ear galls were 5–15% more severe and yield of huitlacoche was 18–150% greater on ears that were not pollinated (i.e., detasseled or male-sterile), compared to those that were pollinated (i.e., not detasseled or hand-pollinated male-sterile). Snetselaar et al (2001) observed that ears that were pollinated 4 d before inoculation developed only 20% smutted kernels, whereas ears that were inoculated 4 d before pollination were 77% smutted. Du Toit and Pataky (1999a) observed that normal pollination reduced the time during which silks were most receptive to infection by U. maydis by 1–4 d, and the incidence of ears with galls was reduced by as much as 50%. The increase in yield of huitlacoche that we observed from unpollinated treatements was not as substantial as expected, based on the results of Snetselaar et al (2001); however, our experimental design did not completely prevent pollination due to contamination by pollen from surrounding fields. Nevertheless, the magnitude of the huitlacoche yield increase and the potential monetary return from controlling pollination was large enough to recommend adoption of this practice when producing huitlacoche commercially.

Additional evaluations of maize cultivars as a medium on which to produce huitlacoche could be valuable because maximum yields of huitlacoche from male-sterile field corn hybrids were about 40% greater than those from sweet corn hybrids. Also, additional evaluations of maize cultivars might corroborate whether the subjective differences in taste of cooked huitlacoche produced on field corn and sweet corn in these studies were due to corn varieties or if they were the consequence of particular growing conditions. Similarly, evaluation of collections of U. maydis might reveal isolates that are best suited for production of huitlacoche.

Based on recent trends in the U.S. and worldwide, the U.S. mushroom industry is expected to continue expanding and diversifying. Some merchandisers have projected steady growth in consumption of specialty mushrooms due to increased consumer awareness and aggressive marketing (Anon. 1999). Huitlacoche represents a unique opportunity for specialty mushrooms growers to develop and expand a potentially profitable niche market. Improved production efficiency from controlling pollination and proper timing of inoculation and harvest further will help develop huitlacoche production systems.

	FOOTNOTES

 
¹ Corresponding author. E-mail: j-pataky{at}uiuc.edu

Accepted for publication April 30, 2003.

	LITERATURE CITED

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