Materials and Methods

Four experiments were done at the Clemson Coastal Research and Education Center in Charleston, SC. Two experiments in Fall 2021 and Spring 2022 were done, one each season in a conventional field and one in a certified organic field. The soil type in all fields was Yonges loamy fine sand, and the soil pH in the organic and conventional fields was 6.5 and 6.8, respectively, in the fall and 6.7 and 6.9 in the spring. Previous crops in the organic fields were ‘Regal Graze’ ladino white clover (Trifolium repens) for 2 years before the Fall 2021 experiment and ‘Regal Graze’ followed by a mixture of rye and Austrian winter pea before the Spring 2022 experiment. Previous crops in the conventional fields were rye, sorghum-Sudan grass hybrid ‘Sudex’, and rye before the Fall 2021 experiment and rye, sorghum-Sudan grass hybrid ‘Sudex’, and balansa clover (Trifolium michelianum) before the Spring 2022 experiment.

In the organic fields, nitrogen [Naturesafe 10-2-8 (Irving, TX), 111 and 185 kg·ha⁻¹ in Fall 2021 and Spring 2022, respectively], potassium (potassium sulfate 0-0-22, 68 and 126 kg·ha⁻¹), phosphorus (bonemeal 0 to 10-0, 147 kg·ha⁻¹) and boron (Solubor, 1.1 kg·ha⁻¹ in Fall 2021 and QB-10 granulated, derived from ulexite, 34% B₂O₃, 4.0 kg·ha⁻¹ in Spring 2022) were incorporated preplant, whereas in the conventional fields 15-0-15 (74 and 102 kg·ha⁻¹ in Fall 2021 and Spring 2022, respectively), potassium (potassium sulfate 0-0-22, 60 and 89 kg·ha⁻¹) and boron (as applied in the organic fields) were incorporated preplant, based on soil tests and fertility recommendations for kale in the southeastern United States (Kemble et al. 2024).

Raised beds 0.9 m wide on 1.8-m centers were covered with white-on-black polyethylene mulch in the fall and black mulch in the spring to manage weeds. Mulches were 0.03-mm thick and 1.5-m wide. A single line of drip irrigation tubing was placed 1 inch below the soil surface in the center of each bed. Plots were single rows, 6.1-m long, separated within rows by 1.5 m of non-planted area. Twenty 5-week-old kale plants of 14 cultivars grown from certified organic seed were transplanted into organic and conventional plots on 8 and 13 Oct 2021 and 15 and 28 Mar 2022, respectively (Table 1, Fig. 1). Plants were spaced 30 cm apart within rows. Plants were fertilized every other week via drip irrigation starting 30 d after transplanting. In both fields in the fall, 2.3 kg sodium nitrate (16N–0P–0K) was applied per week, whereas in the spring, 2.3 kg Allganic 15N–0P–2K was applied per week.

In the organic fields, weeds were managed with caprylic acid 45.14% and capric acid 34.76% (Homeplate) in both seasons, with caprylic acid 47% and capric acid 32% (Suppress) added in the spring. Cabbage worms were controlled using Bacillus thuringiensis 54.0% (XenTari) in both seasons, as well as pyrethrins 5.0% (Pyganic) in the spring. Azadirachtin 1.2% plus pyrethrins 1.4% (Azera) was applied in the fall to manage whiteflies. Sulfur 90.0% (Yellow Jacket) was applied in the spring to manage powdery mildew. In the conventional fields, S-metolachlor (Dual Magnum) was applied preplant at 1.1 and 2.1 kg·ha⁻¹ in fall and spring, respectively, to manage weeds. In the spring, 52 g·ha⁻¹ halosulfuron-methyl (Sandea), 0.14 kg·ha⁻¹ fomesafen (Reflex), and 4% glyphosate (Credit 41 Extra) were added. In the fall experiment, 2.3 L·ha⁻¹ potassium phosphite (Kphite) was applied seven times and 0.54 kg·ha⁻¹ ametoctradin 26.9% + dimethomorph 20.2% (Zampro) was applied three times to manage downy mildew during December to February. To manage aphids, 0.18 kg·ha⁻¹ flupyradifurone (Sivanto) was applied. In the spring, 73 g·ha⁻¹ chlorantraniliprole (Coragen) was applied to manage caterpillars.

All experimental designs were a split-plot with four replications, cultivars as the whole plot, and Alternaria spp. as the subplot. Whole plots were divided in half, and Alternaria spp. were randomly assigned to subplots. The third and fourth last plants on the outer end of each subplot were inoculated, leaving two noninoculated border plants on the outer end of the subplot and six noninoculated plants on the inner end of the subplot (or 12 noninoculated plants between the inoculated plants) (Fig. 2). Five and 3 weeks after transplanting in fall and spring, respectively, selected plants were inoculated with either A. japonica isolate ALT12 or A. brassicicola isolate ALT54. Cultures were grown for 2 weeks on unclarified V8 agar under cool-white, fluorescent lights and 18-h photoperiod at an ambient temperature of 23 °C. A suspension of 5 × 10⁵ conidia/mL was applied with a handheld sprayer to the lowest two leaves on each inoculated plant. To prolong leaf wetness and promote infection, overhead, low-pressure mini-wobbler sprinklers (Senninger Irrigation, Clermont, FL, USA) were used to produce a mist for 15 min each day at 0800 HR and 2000 HR starting the day after inoculation. The morning mist period was omitted on dates any pesticides were applied.

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Harvest date was determined by the length of the largest leaf of curly kale, measured from the tip to the lowest leaflet but excluding the leafless portion of the petiole, because leaves 30 cm long are considered physiologically mature (Reda et al. 2022). Leaves 10 to 30 cm long (based on South Carolina harvest practices) were cut from the two plants inoculated with each isolate and from one plant on each side of the two inoculated plants (Fig. 2). Directly inoculated plants were harvested on 6 to 7 Dec 2021 and 2 Mar 2022 in the organic experiment and 16 to 17 Dec 2021 and 8 Mar 2022 in the conventional experiment (Fig. 2). Leaves on four naturally inoculated plants in the middle of the cultivar plots were harvested on 12 Dec 2021 and 27 Jan 2022 in the organic experiment and 10 Dec 2021 and 13 Jan 2022 in the conventional experiment. In the spring, directly inoculated plants were harvested on 17 and 19 May 2022 in the organic experiment and 2 Jun 2022 in the conventional experiment. (In the conventional experiment, plants were not harvested in one replication due to reduced growth compared with the rest of the field. Directly inoculated plants were not harvested a second time in the spring.) Naturally inoculated plants were harvested on 3 May and 6 Jun 2022 in the organic experiment and 2 and 27 Jun in the conventional experiment. Leaves were sorted as healthy (symptomless) or diseased (one or more lesions of black spot >1 mm diameter), counted, and weighed.

After harvest, two lesions from two leaves on directly inoculated plants and four lesions from naturally inoculated plants were cultured as described previously to determine which Alternaria spp. were present (Keinath 2024).

Results

In all experiments, the main effects of cultivar were highly significant for disease and yield. All factors examined in this study, season, production type, Alternaria spp., and harvest, affected the responses of cultivars to disease or yield (Supplemental Table 1). Three-way interactions with cultivar were common, particularly for natural spread of inoculum.

Disease incidence on kale types and cultivars directly inoculated.

When directly inoculated plants were harvested the first time in three experiments, organic and conventional production in the fall and organic production in the spring, the experiment-by-cultivar interaction was highly significant (Supplemental Table 1). The cultivar-by-pathogen interaction was significant for disease incidence only in fall conventional production (P = 0.03). In this experiment, disease incidence differed between Alternaria spp. only for one cultivar, Tronchuda Beira, the Portuguese kale, which had twice as many diseased leaves when inoculated with A. japonica as with A. brassicicola (Table 2).

Table 2. Incidence of black spot on 14 kale cultivars at the first harvest of mature leaves from plants inoculated with either Alternaria brassicicola or Alternaria japonica in Fall 2022 and Spring 2023.

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Across all three experiments, Portuguese kale had greater disease incidence than lacinato kale (P ≤ 0.01) (Table 2). In the fall conventional and spring organic experiments, Portuguese kale also had more disease than curly and Siberian kales. In both fall experiments, curly kale had more disease than lacinato kale.

In fall conventional production, only Siberian kale cultivars differed within kale type, and only when inoculated with A. japonica. ‘White Russian’ had a greater disease incidence than ‘Russian Royale’, which had disease incidence <1%. In fall organic production, no cultivars differed within kale types. In spring organic production, curly kale cultivars Curly Roja, Darkibor, and Winterbor had lower disease incidence than Oldenbor (Table 2). Siberian kale cultivars separated into two distinct groups: Red Russian and Russian Royale had lower disease incidence than Siberian, Siberian Dwarf, and White Russian.

For organic production, disease incidence was greater in the spring than in the fall for six cultivars: Oldenbor, Toscano, Siberian, Siberian Dwarf, White Russian, and Tronchuda Beira (Table 2).

When regrowth from previously cropped plants that had been directly inoculated was cut at the second harvest in the fall, the maximum disease incidence, 78%, was twice as great as the maximum disease incidence at the first harvest, 34%. All cultivars had disease incidence >20% at the second harvest except Curly Roja, whereas only Tronchuda Beira had this much disease at the first harvest (Tables 2 and 3). Across cultivars and production types, disease incidence was greater on plants inoculated with A. japonica 42.7% ± 1.9% (standard error), than with A. brassicicola, 36.5% ± 1.8%. Disease incidence on inoculated plants differed between production types (significant production-by-cultivar interaction), but only for two cultivars, Black Magic and Red Russian, which had more disease in conventional than organic production.

Some kale types and cultivars differed from each other. Similar to the first harvest, Portuguese kale had greater disease incidence than the other three kale types in both production systems (Table 3). In addition, Siberian kale had greater disease incidence than lacinato kale. In both production types, Curly Roja had lower disease incidence than all other curly kale cultivars. In organic production, Oldenbor had greater disease incidence than all other curly kale cultivars. Within Siberian kales, cultivar Red Russian had lower disease incidence than White Russian in organic production, and Siberian Dwarf had lower disease incidence than White Russian in conventional production.

Table 3. Incidence of black spot on regrowth of 14 kale cultivars at the second harvest on plants directly inoculated or naturally inoculated with Alternaria brassicicola or Alternaria japonica in Fall 2022.

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Yields of kale types and cultivars directly inoculated.

Marketable yield of kale types and cultivars directly inoculated with A. brassicicola or A. japonica was measured for one harvest of mature leaves in three experiments, organic and conventional production in the fall and organic production in the spring. The main effect of pathogen species was significant (P = 0.0121), whereas the cultivar-by-pathogen interaction was not (Table 2). Mean marketable weight was significantly lower for plants inoculated with A. japonica, 771 ± 18 cartons/ha, than for plants inoculated with A. brassicicola, 814 ± 18 cartons/ha.

The experiment-by-cultivar interaction was highly significant (P = 0.0001) (Supplemental Table 1). When the experiments were analyzed separately, pathogen species had no effects on yields, and the cultivar-by-pathogen interactions were not significant (P ≥ 0.07) (Table 4).

Table 4. Yield of 14 kale cultivars affected with black spot at the first harvest of mature leaves from plants inoculated with either Alternaria brassicicola or Alternaria japonica in Fall 2022 and Spring 2023.

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In the fall experiments, Siberian kales and Portuguese kale yielded more than curly and lacinato kales. In the spring organic experiment, however, curly kales yielded more than Siberian kales. In both fall experiments, the hairy Siberian kale cultivars Siberian and Siberian Dwarf yielded more than smooth-leaved cultivars, but in the spring organic experiment, the reverse was observed. However, the smooth-leaved cultivar White Russian yielded as poorly as the two hairy-leaved cultivars. In the fall, cultivars Siberian and Siberian Dwarf yielded more than the other four Siberian kale cultivars in the organic and conventional experiments, respectively, while Red Russian and Russian Royale were among the lowest yielding Siberian cultivars. In the spring, the converse was observed, Red Russian and Russian Royale yielded more than Siberian, Siberian Dwarf, and White Russian.

In the fall experiments, the red curly kale ‘Curly Roja’ had a lower yield than all other curly cultivars in the organic field and a lower yield than ‘Blue Curled Scotch’ in the conventional field. In the spring organic experiment, cultivars Darkibor and Winterbor yielded more than Curly Roja and Oldenbor.

Disease incidence on kale types and cultivars naturally inoculated.

At the first harvest of mature leaves, no black spot symptoms were found on noninoculated plants in either the organic or conventional experiments in Fall 2022. On the regrowth in the fall experiments, very little disease, <2% incidence, developed on naturally inoculated plants in the organic field, so comparisons among kale types and cultivars reflect disease development in the conventional field. Portuguese kale had a greater incidence than the other three kale types and all other cultivars (Table 3). Curly kales had more disease than lacinato kales. Within curly kales, cultivars Curly Roja and Winterbor had lower disease incidence than Oldenbor and Blue Curled Scotch. Within Siberian kales, cultivar Russian Royale had lower incidence than Siberian.

In Spring 2023, the production-by-cultivar interaction was not significant (P = 0.58), and disease incidence was slightly greater in conventional production, 10.2%, than in organic production, 6.9% (P = 0.037). Portuguese kale was more susceptible than the other three kale types. Cultivars differed only within curly kales, where Curly Roja and Winterbor again had lower disease incidence than Oldenbor (Table 3).

Yields of kale types and cultivars naturally inoculated.

In both fall and spring, naturally inoculated plants yielded an overall mean across cultivars of 1.95 kg leaves per 1.3 m, or ∼1075 boxes/ha at the first harvest, and a quarter of that, 0.52 kg/1.3 m or 286 boxes/ha, at the second harvest.

In both seasons, kale types differed in yield at the first harvest when they were exposed to natural spread of inoculum of Alternaria spp. In the fall, there was no production type-by-cultivar interaction (P = 0.83), but in the spring this interaction was significant (P = 0.0053), because two curly kale cultivars and all three lacinato cultivars yielded more in the organic field than in the conventional field (Table 5).

Table 5. Yield of healthy leaves at the first harvest of 14 kale cultivars with black spot due to natural spread of inoculum.

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In the fall, Siberian and Portuguese kales yielded more than curly and lacinato kales. Among Siberian kales, hairy cultivars yielded more than smooth-leaved cultivars. Cultivars Siberian, Siberian Dwarf, and White Russian had greater yields than Red Russian and Russian Royale. In addition, ‘Red Russian’ yielded more than ‘Russian Royale’. Curly kales yielded more than lacinato kales. Among curly kales, Curly Roja had a lower yield than all other cultivars, and its yield also did not differ from the yield of any lacinato kale cultivar.

In the spring, Siberian kales yielded more than the other three kale types, which did not differ among each other. In addition, the hairy Siberian cultivars yielded more than the smooth-leaved types. Siberian Dwarf yielded significantly more than the other four Siberian cultivars.

Curly kale cultivars differed in both production types. ‘Curly Roja’ had low yields in both production types, lower than all other curly kale cultivars in the organic field and lower than cultivars Darkibor and Winterbor in the conventional field. In addition, ‘Winterbor’ yielded more than ‘Oldenbor’ in the conventional field.

Regrowth from plants cropped at the first harvest was harvested in the second harvest (Table 6). In the fall, the production type-by-cultivar interaction (P = 0.0001) was due to significant differences between production types for four cultivars. Cultivars Curly Roja and Russian Royale yielded more in the conventional field, whereas Siberian Dwarf and Tronchuda Beira yielded more in the organic field. In the spring, the production type main effect was significant but not the interaction. Mean yield in the organic field was 251 cartons/ha, but mean yield was only 89 cartons/ha in the conventional field, where plants appeared smaller than in the conventional field for unknown reasons.

Table 6. Yield of healthy leaves at the second harvest of 14 kale cultivars with black spot due to natural spread of inoculum of Alternaria species.

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In the fall organic and both spring experiments, Siberian kales yielded more than curly kales. In the fall conventional and both spring experiments, Siberian and lacinato kales yielded more than Portuguese kale. Among Siberian kales, cultivars Red Russian, White Russian, and Siberian yielded more than Russian Royale in the fall organic experiment. In the spring experiments, ‘Siberian’ yielded more than ‘White Russian’. In fall organic production, ‘Toscano’ yielded more than ‘Black Magic’, the only difference between lacinato cultivars observed in the study.

In the fall conventional experiment, curly kale cultivars Darkibor and Curly Roja yielded more than Oldenbor. In both production types in the spring, ‘Oldenbor’ had a lower yield than the other five curly kale cultivars due to high incidence of black spot. Cultivars Winterbor, Darkibor, and Blue Curled Scotch yielded more than Curly Roja.

Overall effects of kale types and cultivars.

Because all factors interacted with cultivar (Supplemental Table 1), season, production type, and pathogen species were considered random effects in a final analysis to make conclusions about susceptibility and yield potential of cultivars affected with black spot. (Disease incidence and yield were summed across the two harvests for this analysis.) Portuguese kale had greater disease incidence than all other kale types and all other kale cultivars with both natural and direct inoculations (Fig. 3). In addition, curly kales had greater disease incidence than lacinato kales. With natural spread of inoculum, curly kale cultivars Curly Roja and Winterbor had lower disease incidence than Darkibor, and Curly Roja also had lower disease incidence than Oldenbor, but curly kale cultivars did not differ when directly inoculated. Similarly, with natural spread of inoculum but not direct inoculation, Siberian kale cultivar Russian Royale had lower disease incidence than the other Siberian cultivars except Red Russian, which had lower disease incidence than White Russian and Siberian Dwarf.

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With natural spread of inoculum, ‘Winterbor’ curly kale and four of the Siberian kale cultivars yielded >1000 cartons/ha at the row spacing used in this study, but no cultivars did with direct inoculation (Fig. 4). Siberian kales yielded more than curly and lacinato kales with both inoculations. Red curly kale cultivar Curly Roja had the lowest yields, significantly lower than green kale cultivars Winterbor and Blue Curled Scotch. With natural spread of inoculum, cultivars Siberian and Siberian Dwarf had greater yields than Russian Royale, but yields of Siberian kale cultivars did not differ with direct inoculation.

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Recovery of Alternaria spp.

In the fall at the first harvest of inoculated plants in the organic field, both species were recovered at a high percentage, mainly from plants inoculated with that species. A. brassicicola and A. japonica were recovered from 73% and 86%, respectively, of subplots inoculated with each species, and from 4% and 2%, respectively, of subplots inoculated with the other species. Neither species was recovered from 15% of the subplots. By the second harvest, both species were recovered from most inoculated plants, although A. japonica was recovered from 71% of subplots inoculated with A. japonica and 55% of subplots inoculated with A. brassicicola, and A. brassicicola was recovered from 45% of subplots inoculated with A. brassicicola and 27% of subplots inoculated with A. japonica. Neither species was recovered from 12% of the subplots. No diseased leaves were found on naturally inoculated plants at the first harvest, and, because only five diseased leaves were found at the second harvest in the organic field, lesions were not cultured.

In the conventional field in the fall, both species also were recovered primarily from the plants inoculated with that species. A. brassicicola and A. japonica were recovered from 84% and 90% of subplots, respectively, inoculated with each species, and from 2% and no subplots inoculated with the other species. Neither species was recovered from 25% of the subplots. By the second harvest, A. japonica was the dominant species on all plants harvested, recovered from 96% and 77% of the subplots inoculated with A. japonica and A. brassicicola, respectively. A. brassicicola was recovered from 34% of the subplots inoculated with A. brassicicola and 20% of subplots with A. japonica. Only two subplots yielded neither species. On naturally inoculated plants, no leaves had black spot lesions at the first harvest. At the second harvest, A. japonica also was the dominant species, recovered from 55% of the subplots, whereas A. brassicicola was recovered from 27% of the subplots, and 41% of the subplots yielded neither species.

In the spring, recovery was low from plants in the organic field directly inoculated, as only 39% and 21% of the subplots yielded A. brassicicola and A. japonica, respectively, when inoculated with these species. A. brassicicola and A. japonica were also recovered from 7% and 11%, respectively, of the subplots inoculated with the other species. Neither species was recovered from 52% of the subplots. Recovery also was low from naturally inoculated plants in this field, as each species was recovered from only 13% of the subplots, and only together in one of 132 subplots. Neither species was found in 77% of the subplots. Lesions were not cultured in the conventional field in spring.

Discussion

In all experiments, the main effects of cultivar were highly significant for disease and yield. All factors examined in this study, season, production type, Alternaria spp., and harvest, affected the responses of cultivars to disease or yield. Three-way interactions with cultivar were common, particularly for natural spread of inoculum (Supplemental Table 1). As a group, Siberian kales yielded more overall than curly and lacinato kales. The same pattern of differences in yields of kale types was observed in a Colorado study with cultivars Red Russian, Winterbor, and Dinosaur as representatives of the three kale types, respectively (Yoder and Davis 2020). In Kentucky, three of the Siberian kale cultivars tested in the current study, Siberian, White Russian, and Red Russian, also yielded more than curly and lacinato kale cultivars in the absence of foliar diseases (Coolong et al. 2013). In that study, conducted in spring and fall seasons in two years, year and season also interacted with cultivar.

Among the five curly kale cultivars tested in this study, Winterbor was notable for its tendency to have lower incidence of black spot and higher marketable yields. It was the only curly kale cultivar to yield >1000 cartons/ha across two harvests. In all experiments with natural spread of Alternaria inoculum, ‘Winterbor’ had lower incidence and greater yields than ‘Darkibor’, the most widely grown curly kale cultivar in South Carolina (Fig. 4). ‘Winterbor’ appears to be a good choice for kale production in South Carolina and other parts of the southeastern United States, as it also was the most productive cultivar of four curly kales evaluated in organic production in Kentucky (Coolong et al. 2013).

In general, red curly kale cultivar Curly Roja yielded the least, not just among curly kale cultivars but across all 14 cultivars. Plants and leaves were shorter than other cultivars, even at low disease incidence. Another red curly kale, ‘Redbor’, also yielded the least among not only curly kales but among all kale cultivars and types tested in organic production in Kentucky (Coolong et al. 2013). Red curly kale is used in chopped, bagged kale mixes to add color and interest, but other than this specific use, it is unlikely that ‘Curly Roja’ would yield enough to produce an acceptable net return, even though it was one of the least susceptible cultivars with natural spread of inoculum.

Green curly kale cultivar Oldenbor tended to have greater disease incidence than other curly kale cultivars when cultivar differences were observed. ‘Oldenbor’ is grown in South Carolina primarily for early fall harvest because it is advertised as having resistance to yellows, which is critical when kale is planted in summer into warm soils infested with Fusarium oxysporum f. sp. conglutinans (Farnham et al. 2001). Because black spot can be managed with fungicides, whereas yellows cannot, growers will likely continue to plant ‘Oldenbor’ despite its increased susceptibility to black spot, in part because yields of ‘Oldenbor’ were at least as good as most other curly kale cultivars (Dutta et al. 2020).

The commonly cultivated Siberian kale, ‘Red Russian’, showed average to below average disease incidence, even when directly inoculated, and average to above average marketable yields overall. Russian Royale, a morphologically similar cultivar with shiny red leaves, also was less susceptible than many other cultivars but had slightly lower yields than ‘Red Russian’. Thus, ‘Red Russian’ is likely the best Siberian kale for most producers. In organic production in Kentucky, it was among the most productive cultivars overall (Coolong et al. 2013). Because it consistently yielded more than ‘Curly Roja’, it could be used to provide color in kale salad mixes.

The Siberian kale cultivars Siberian and Siberian Dwarf, which had similar morphology, developed fine hairs on the leaf blades and petioles once leaves matured. Hairy leaves are also typical of wild B. oleracea spp. (Cai et al. 2022). Although these two cultivars produced a large marketable biomass in experiments with natural inoculum spread, the hairs may make these cultivars unsuitable for fresh markets. These two cultivars also had relatively high percentages of diseased leaves when plants were directly inoculated.

The Portuguese kale cultivar Tronchuda Beira was included to increase the types of kale represented in this study. Its morphology was decidedly unlike kale and more like collard (B. oleracea var. viridis) because the wide, flat light green leaves had entire margins (Cai et al. 2022; Hahn et al. 2022). It was uniformly the most susceptible cultivar in this study and, consequently, had low marketable weight. This cultivar, the only Portuguese kale we found, is not recommended where black spot is a risk.

None of the 14 cultivars could be considered tolerant (Pagán and García-Arenal 2018; Schafer 1971). Tolerance in kale could occur, for example, if black spot affected only the lowest leaves on certain cultivars, and more midsized leaves remained healthy. Although some cultivars (e.g., Winterbor) tended to have low disease incidence and high yields in some experiments, there was no overall correlation between disease and yield (data not shown). The concept of tolerance may be more applicable to fruiting vegetables, where disease occurs directly on the consumed portion of the crop, than leafy vegetables.

Main effects of season were significant for incidence of black spot with both direct and indirect inoculations, but season was not a significant source of variation for marketable yield. Black spot was more prevalent in spring than in fall, particularly on directly inoculated plants. The main environmental difference between spring and fall was warmer air temperature in the spring, because misting provided leaf wetness in both seasons.

Main effects of production type were significant with natural spread of inoculum but not with direct inoculation. Apparently, the greater concentration of inoculum when plants were sprayed with a conidial suspension was a more important factor affecting black spot than the indirect effects of production type. When differences between production types were observed, disease incidence was slightly greater in conventional production, and marketable weight was greater in organic production for first and second harvests. In each production type, rates of fertilizers were calculated to deliver 22.7 kg·ha⁻¹ nitrogen and 45.4 kg·ha⁻¹ potassium (Kemble et al. 2024). The main difference, therefore, between organic and conventional production was the form of nitrogen added preplant. The organic granular fertilizer included nitrogen derived from feather meal, meat and bone meal, and blood meal, which, according to the manufacturer, was slowly available over 7 to 8 weeks. The conventional fertilizer supplied nitrogen as ammonium sulfate, urea, and diammonium phosphate, the ammonium being water-soluble. In previous studies, conventional nitrogen applications reduced severity of Alternaria leaf diseases on field-grown carrot, potato, and cotton (Abuley et al. 2019; Blachinski et al. 1996; Westerveld et al. 2008). Foliar concentrations of nitrogen were not measured in this study. It is possible, though, that the more persistent organic nitrogen sources increased foliar nitrogen content compared with the more readily available but less persistent conventional nitrogen sources. Overall, organic production was less conducive to black spot and had a beneficial effect on marketable yield of kale.

Across three experiments, Alternaria spp. differentially affected yield, but disease incidence was similar between A. brassicicola and A. japonica (Tables 2 and 7). In a previously published study, incidence of black spot caused by A. brassicicola or A. japonica also did not differ, and only minor effects on yield were observed (Keinath 2024). A. japonica tolerates lower minimum temperatures than A. brassicicola for conidial germination (13 vs. 15 °C, respectively), infection (17 vs. 19 °C), and subsequent sporulation on infected tissue (13 vs. 19 °C) (Degenhardt et al. 1982). This tolerance of lower temperatures may explain why A. japonica was recovered from a greater percentage of plots in the fall than A. brassicicola was. Temperatures in the fall experiments, however, seem to have been favorable for both species to cause black spot. Thus, the presence of A. japonica in the southeastern United States does not necessarily mean that black spot will be more problematic in cool growing conditions (i.e., on overwintered crops) than in warm weather.

Table 7. Overall mean susceptibility to black spot and overall mean yield of kale cultivars exposed to inoculum of Alternaria brassicicola or Alternaria japonica and grown with organic and conventional production in Fall 2022 and Spring 2023.ⁱ

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Strictly grading leaves into symptomless and symptomatic categories was based on the low tolerance for decay in wholesale markets and grocery stores, 1% per USDA standards for kale, and repeated observations that black spot can develop on symptomless leaves when they were stored at high relative humidity at standard postharvest temperatures, ≈4.4 °C (Keinath, Silva, and Turner, unpublished data) (Johnson et al. 2018; US Department of Agriculture, Agricultural Marketing Service 2008). In addition, A. brassicicola was recovered from several diseased pieces of commercial chopped, bagged kale stored beyond the recommended interval (Toporek et al. 2024). Yields of some Siberian kale cultivars could have been greater if ≤ % discoloration due to black spot had been considered marketable, as black spot lesions tended to be smaller on Siberian kales than other kale types. Even with strict grading, the yields of Siberian kales often were greater than yields of other kale types, so it is unlikely that a more lenient grading scale would have affected these results.

When plants were harvested a second time, disease incidence was noticeably greater and yields were much lower than at the first harvest, even though the leaves harvested were about the same age as the leaves cropped at the first harvest. It is well known that older leaves of brassicas are more susceptible to Alternaria spp. than younger leaves (Keinath 2024; Tewari and Buchwaldt 2007). The regrowth leaves, however, seemed to be physiologically as old as the plants from which they were cut, based on their susceptibility to black spot. Growers should be aware of the increased risk of black spot on leaves produced by cropped plants and adjust their fungicide applications accordingly (e.g., use 7-d application intervals, apply the maximum rate, or choose fungicides that are the most effective) (Dutta et al. 2020).