Augustinegrass Cultivars Research Articles

Molecular analysis of St. Augustinegrass cultivar mixtures composition over time and latitude

AbstractSt. Augustinegrass [Stenotaphrum secundatum (Walt.) Kuntze] is commonly planted in residential and commercial landscapes as a cultivar monoculture predisposing this lawn to pest invasion and high‐maintenance inputs. Researchers have suggested that increasing genetic diversity by growing cultivars in mixtures may increase turfgrass stress resilience. However, the stability and uniformity of those mixtures has not been studied. The present study was carried out to evaluate the stability of St. Augustinegrass cultivars mixtures over time and across three latitudes. The study was conducted in Citra and Fort Lauderdale, FL, and Jackson Springs, NC. Simple‐sequence repeats markers were used to genotype leaf samples of St. Augustinegrass cultivars planted in two‐ and four‐cultivar mixtures. Leaf samples were collected 1 and 3 years after establishment. In all locations, cultivar richness and evenness declined over time. Similarly, the relative abundance of the least persistent cultivars decreased approximately 50%–100% depending on cultivar and location. Differences in growth patterns among cultivars resulted in cultivar displacement and the predominance of a single cultivar. Cultivars that covered the ground faster or formed dense canopies early after establishment were dominant at the end of the study. Locally developed cultivars tended to be more dominant in their original latitude. The use of cultivar mixtures may help the identification of vigorous, competitive, and stress tolerant cultivars in turfgrass breeding programs. However, their commercial use remains challenging as if the patterns observed here for 3 years are representative of a continuous trend, and they do not persist over time.

Mixing warm‐season turfgrass cultivars to reduce weed pressure and increase lawn quality

AbstractSt. Augustinegrass [Stenotaphrum secundatum (Walter) Kuntze] is a warm‐season turfgrass widely planted in monoculture throughout tropical and subtropical regions of the world. Monocultures preserve desirable inheritable traits but may predispose turfgrass lawns to pest invasion and reliance on high‐maintenance inputs. Evidence from multiple plant systems suggests that increasing inter‐ or intra‐specific plant diversity increases resilience to biotic and abiotic stress. Here, we tested if mixing St. Augustinegrass cultivars would be a viable approach to reducing weed invasion and increasing turfgrass lawn quality compared with cultivar monocultures. Mixed‐cultivar plantings grew into denser stands of grass, which translated to about 4% less weed cover than cultivar monocultures. Digital image analysis showed that mixtures of four cultivars grew into higher quality turfgrass stands than cultivar monocultures. This study provides novel evidence that cultivar blends may reduce weed pressure and enhance St. Augustinegrass lawn quality, which, combined with other potential resilience benefits, may create a more sustainable approach to warm‐season turfgrass lawn production and management.

Characterizing the growth and winter survival of commercially available and experimental genotypes of St. Augustinegrass

AbstractSt. Augustinegrass [Stenotaphrum secundatum (Walter) Kuntze] is a commonly used warm‐season turfgrass for lawns in warm‐humid to tropical climates. Efforts to breed improved cultivars with enhanced abiotic and biotic stress tolerance are ongoing, but additional data on their growth and adaptation are needed. Twenty‐one commercially available St. Augustinegrass cultivars and eight experimental genotypes were planted in the field in Fayetteville, AR, in both 2009 and 2010 to study their stolon, leaf, and establishment characteristics, as well as winter survival. ‘Floratam’, ‘Floralawn’, ‘FX‐10’, ‘FA‐40’ (Mercedes™), ‘B12’ (Sapphire®), and WS had the highest stolon growth rate (>8.9 mm d−1), whereas ‘TR 6‐10’ (Amerishade®), ‘6‐72‐99’ (Delmar™), ‘6‐72‐182’ (Jade™), ‘6‐72‐130’ (Sunclipse™), 106G3, 106T3, and SV27 had the lowest stolon growth rate (<5.1 mm d−1). Cultivars ‘Floraverde’, Floralawn, Floratam, B12 (Sapphire), and ‘Texas Common’ were the quickest to establish. Principal component analysis (PCA) identified that cultivars with similar ploidy levels had similar growth characteristics as increasing ploidy resulted in wider and longer leaves, as well as larger stolon diameter and internode length. Cultivars considered dwarf types with short stolons or narrow‐short leaves grouped separately from cultivars with long stolons or wide‐long leaves. Genotypes VNS (Classic™), GF, VNS (Majestic™), ‘SS‐100’ (Palmetto®), ‘Raleigh’, TAES 5714, and Texas Common showed the highest winter survival and associated coverage after winter in both years. Results from this study are intended to help turfgrass breeders, turf producers, practitioners, and homeowners make informed cultivar selection decisions. Planting well‐adapted cultivars will reduce reestablishment costs from winter‐kill and ultimately increase sustainability.

Long‐term effects of low N rates on St. Augustinegrass cultivars

AbstractCurrent recommended nitrogen fertilization rates for St. Augustinegrass [Stenotaphrum secundatum (Walter) Kuntze] in South Florida range from 4 to 6 lb N per 1,000 ft2 per yr, applied throughout the year. Nevertheless, strict fertilizer ordinances are being passed by multiple counties in Florida, setting limits to the amount of N that can be applied to turfgrass areas, and establishing summer blackout periods for fertilizer applications; hence, there is a need to identify the minimum N rates to sustain St. Augustinegrass quality. A 4‐yr study was conducted at the University of Florida's Fort Lauderdale Research and Education Center to assess the turfgrass quality of four St. Augustinegrass cultivars (‘CitraBlue’, ‘Floratam’, ‘Palmetto’, and ‘Raleigh’) grown on a Hallandale fine sand (siliceous, isohyperthermic Lithic Psammaquents) and fertilized at either 2.5 or 5 lb per 1,000 ft2 per yr. Plots were evaluated monthly for visual turfgrass quality, and yearly for genetic color, leaf texture, spring green‐up, and summer density. CitraBlue had the highest turfgrass quality, color, and density. Differences were rarely detected among the other three cultivars. Nitrogen rates had an effect only on summer density, suggesting that St. Augustinegrass grown on Hallandale fine sand with 3.4% organic matter in South Florida could be managed with less N than is currently recommended without detrimental effects.

Response of St. Augustinegrass cultivars to pinoxaden alone and in combination with select fungicides

AbstractPinoxaden is a herbicide inhibiting acetyl coenzyme A carboxylase recently introduced for use in turfgrass. Currently, pinoxaden is labeled for use in all bermudagrass [Cynodon dactylon (L.) Pers.] and zoysiagrass (Zoysia spp.) turfgrass areas except putting greens. However, pinoxaden is only labeled for use in St. Augustinegrass [Stenotaphrum secundatum (Walter) Kuntze] sod production because of injury concerns. Tank mixtures of two or more herbicides can reduce injury to turfgrass species. Injury reduction can also occur when fungicides are tank‐mixed with herbicides. Research was conducted between 2018 and 2019 to determine the effect that pinoxaden alone and tank mixtures of pinoxaden with one of two common fungicides, chlorothalonil and propiconazole, would have on four different cultivars of St. Augustinegrass: ‘Classic’, ‘Floratam’, ‘Palmetto’, and ‘Raleigh’. At the conclusion of the study, we determined that there is differential susceptibility to pinoxaden between cultivars of St. Augustinegrass and that both chlorothalonil and propiconazole have a safening effect on pinoxaden applications to St Augustinegrass, regardless of cultivar. The cultivars Classic and Raleigh were more susceptible to pinoxaden than Floratam and Palmetto. All four cultivars were less susceptible to pinoxaden when chlorothalonil and propiconazole were applied in combination with pinoxaden. These results suggest that pinoxaden can be applied in a tank mixture with either chlorothalonil or propiconazole to reduce pinoxaden injury to St. Augustinegrass. However, the effect that these tank mixtures may have on weed control is currently unknown.

Registration of ‘MSA 2‐3‐98’ St. Augustinegrass

‘MSA 2‐3‐98’ (Reg. No. CV‐284, PI 689102) is a cold‐tolerant, high‐quality St. Augustinegrass [Stenotaphrum secundatum (Walt.) Kuntze] developed at Mississippi State University. MSA 2‐3‐98 was released on 19 Apr. 2006 from Mississippi Agricultural and Forestry Experiment Station. MSA 2‐3‐98 offers many advantages compared with other St. Augustinegrass cultivars. Leaf length, leaf width, internode length, and internode diameter of MSA 2‐3‐98 were significantly smaller than ‘Floratam’ and ‘Raleigh’. The more diminutive morphology of MSA 2‐3‐98 allows it to establish a denser turfgrass canopy with finer leaf texture. Cold tolerance of MSA 2‐3‐98 was evaluated from spring assessments of percentage green turfgrass cover and overall plot green‐up. The cold tolerance of MSA 2‐3‐98 was similar to Raleigh but outperformed Floratam. MSA 2‐3‐98 turfgrass quality tended to be greater than ‘Delmar’, Floratam, and Raleigh. Gray leaf spot severity on MSA 2‐3‐98 was variable among locations where the disease was evaluated. MSA 2‐3‐98 has greater turfgrass quality than Raleigh, with similar cold tolerance.

Efficiency of Embryo Rescue Technology in St. Augustinegrass Breeding

St. Augustinegrass [Stenotaphrum secundatum (Walt.) Kuntze] is a popular warm‐season turfgrass in Gulf Coast regions of the United States. Diploid St. Augustinegrass produces fertile seed, but triploids and higher polyploids are mostly sterile. Polyploid St. Augustinegrass cultivars have been selected for their resistance to southern chinch bugs, St. Augustine Decline, and drought tolerance but lack the cold hardiness and shade tolerance of diploids. However, because of sterility barriers, the polyploid germplasm had not been fully used as breeding material until 2009 when we reported the development of interploid hybrids using embryo rescue technology. The objective of the present study was to backcross these first‐generation interploid hybrids with diploid males to produce second‐generation hybrids. Sterility barriers were found to be much greater with second‐generation crosses yielding only 0.00 to 2.69% progeny per interploid hybrid. Using this technique, the embryos are generally rescued 3 wk postpollination, thereby causing a heavy demand on one's time to process spikelets before embryos abort. Our preliminary findings show the use of cold storage at 7°C to potentially extend the processing period up to 2 mo with a slight drop off in efficiency (4.37–6.03% crossability). Our results also show the potential of first‐generation hybrids to exhibit superior performance under drought when compared with diploid cultivars Palmetto and Raleigh. Tested second‐generation interploids did not exhibit superior performance in drought. Nonetheless, our results emphasize the utility of embryo rescue technology in facilitating gene exchange between ploidy levels to generate hybrids with superior performance.

Interspecific Hybrids between Pembagrass and St. Augustinegrass

St. Augustinegrass [Stenotaphrum secundatum (Walt) Kuntze] is an important turfgrass throughout many of the warm, humid regions of the world, including the southeastern United States. Unfortunately, the grass is susceptible to a number of insects, pathogens, and nematodes that adversely reduce its desirability as a turfgrass. Another Stenotaphrum species, pembagrass [S. dimidiatum (L.) Brongn.], offers a source of resistance to some of these pests. Crosses were made between pembagrass and St. Augustinegrass in an attempt to transfer desirable resistance genes into St. Augustinegrass. Two pembagrass accessions (PI 289729 and PI 365031) were used as the maternal parents, and different St. Augustinegrass cultivars and breeding lines were used as the paternal parents. Seed from these crosses were small, shriveled, lacking fully developed endosperm, and unable to germinate. A total of 218 embryos were cultured on a growth medium using embryo rescue techniques, and 127 seedlings were recovered. Paternal‐specific expressed sequence tag‐simple sequence repeat markers were used to identify the hybrids, and all 127 seedlings were determined to be F1 hybrids. These are the first reported hybrids between pembagrass and St. Augustinegrass. The crossability between the two species ranged from 0 to 67.1%, depending on the genotypes used, and the overall mean crossability was 30.8%. Crossability was greatly influenced by which pembagrass genotype was used as the maternal parent. This is an initial step toward introgressing desired genes for insect, nematode, and pathogen resistance from pembagrass into St. Augustinegrass.

Gamma‐Ray Irradiation Improves Turfgrass Characteristics of St. Augustinegrass

St. Augustinegrass [Stenotaphrum secundatum (Walt.) Kuntze] is a warm‐season turfgrass with medium to high shade tolerance, a valuable trait for use in lawns, particularly in smaller residential landscapes and other green spaces where trees are dominant. However, their long internodes and very coarse leaf and stolon texture are undesirable in home lawns and public spaces. Gamma (γ) irradiation has been used to induce useful variations for various morphological traits in turfgrass breeding. The objective of the present study was to induce dwarf and semidwarf phenotypes from two St. Augustine germplasms, S1 and S2, irradiated with 50, 100, 150, or 300 Gy and 20, 40, 80 or 120 Gy respectively, using a 60Co source. The lethal does (LD50) for S1 and S2 genotypes were 95 and 103 Gy, respectively. The linear reduction of survival rate with increasing gamma rays was highly correlated (r2 = 0.96 and 0.87). The 15 morphological mutants (0.35% of the irradiated plants) with a semidwarfed growth habits were identified among 4301 node cuttings. The mutant lines exhibited up to 40% shorter plant height and finer leaf and stolon texture than parental genotypes. Mutant lines developed in this study may offer improved St. Augustinegrass cultivars for landscaping.

Turf Performance of Bahiagrass, Centipedegrass, and St. Augustinegrass Cultivars under a Linear Gradient Irrigation System

Periodic drought, irrigation requirements, and the enforcement of mandatory watering restrictions have posed a challenge for homeowners and landscape managers to maintain acceptable turf quality during dry periods. Information regarding irrigation requirements and performance of common turfgrass species used for residential and commercial landscapes in the southeastern United States would be valuable. A Linear Gradient Irrigation System (LGIS) provides an efficient way to measure cultivar responses to variable moisture regimes. Nine commercially available cultivars from three turfgrass species: ‘Argentine’ bahiagrass (BH) (Paspalum notatum Flügge); ‘Common’ and ‘TifBlair’ centipedegrass (CP) [Eremochloa ophiruoides (Munro) Hack.]; ‘Captiva’, ‘Classic’, ‘Floratam’, ‘Palmetto’, ‘Raleigh’, and ‘Sapphire’ st. augustinegrass (STA) [Stenotaphrum secundatum (Walt.) Kuntze.] were evaluated during naturally occurring dry periods in 2010 and 2011 for visual quality, color, and density and chlorophyll index. An irrigation gradient ranging from 0% to 120% of reference evapotranspiration (ETo) was provided by LGIS. Centipedegrass and BH had less demands on supplemental irrigation to maintain acceptable quality during periodic drought compared with STA. ‘Argentine’ BH performed similar to ‘Common’ CP for its visual quality except at the irrigation level of 40% ETo. Among STA cultivars, ‘Palmetto’ performed poorest in its relative drought response than other cultivars. ‘Sapphire’ STA needs further study to better characterize its drought response. The irrigation level of 120% ETo decreased turf quality except for ‘Argentine’ BH, and the irrigation replacement at 40% to 80% ETo, depending on turfgrass species and cultivar, may provide enough supplemental moisture to maintain acceptable turf quality during short-duration drought (≈2 weeks) in north Florida regions.

Developmental Response of St. Augustinegrass Cultivars and Experimental Lines in Moderate and Heavy Shade

St. augustinegrass [Stenotaphrum secundatum (Walt.) Kuntze] is considered to be one of the most shade-tolerant warm-season turfgrasses, yet information is lacking on intraspecies developmental responses and performance in shade. This greenhouse study was conducted to 1) compare quality, development, and physiological responses of 10 commercial and experimental lines of st. augustinegrass in moderate and heavy [32% and 15% photosynthetic photon flux (PPF), respectively] shade environments’ and 2) evaluate physiological and morphological indicators that could be used in rapid screening for shade tolerance among st. augustinegrass progeny from a segregating population. A range of shade tolerance was observed between the entries, as noted by quality and percent green cover after 10 weeks of imposed shade conditions. In moderate shade, most entries maintained acceptable (6 or greater) quality and greater than 50% green cover. However, in heavy shade, only ‘Captiva’, ‘Amerishade’, and ‘PI 600734’ maintained acceptable quality, with only PI 600734 and Captiva maintaining greater than 50% cover. ‘TAES 5732-6’, an embryo rescue-derived hybrid from ‘Floratam’, exhibited the least shade tolerance of the group in both shade environments. Neither chlorophyll content nor total nonstructural carbohydrates related well to observed shade quality differences between the entries. A strong correlation existed between shoot elongation rate of a cultivar and its corresponding final percent green cover in moderate shade (R2 = 0.66) but not in heavy shade (R2 = 0.19), suggesting that moderate shade may be the better environment for discriminating genetic differences among st. augustinegrass germplasm for shade tolerance.

Morphological and Physiological Responses of St. Augustine Grass Cultivars to Different Levels of Soil Moisture

Understanding responses of turf grasses to drought stress is important for water resource management to maintain an acceptable level of quality for turfs under prolonged drought conditions. Four cultivars of St. Augustine grass (SA) (Stenotaphrum secundatum [Walt.] Kuntze) were evaluated for morphological and physiological responses to four watering treatments in greenhouse studies. Soil moisture treatments had greater impact on stolon number, photosynthetic rate, and dry matter production than on leaf sheath length and chlorophyll fluorescence. Full irrigation and the 75% field capacity (FC) did not result in significant differences in most of the characteristics, whereas the 25% FC significantly reduced morphological growth, physiological activities, and dry matter production of cultivars. The time when the morphological characteristics started showing differences among the watering treatments varied, with stolon numbers beginning to show response to watering treatments at week 2 (two weeks after watering treatment) and leaf sheath length not differing significantly until week 5 (five weeks after watering treatments). Cultivars differed in response to soil watering treatments among the characteristics. Floratam had the longest stolons, largest leaves, and greatest dry matter accumulation among cultivars across the environments. Raleigh had the lowest dry matter production at all levels of soil moisture but the least reduction of dry matter at 25% FC.

Drought Response and Recovery Characteristics of St. Augustinegrass Cultivars

ABSTRACTAs water resources become restricted for use on amenity turfgrass systems, the inability for consumers to delineate incremental drought stress relating to plant health can result in the misuse of water resources during drought conditions. Seven cultivars of St. Augustinegrass (SA) [Stenotaphrum secundatum (Walt.) Kuntze] and two root zone depths were evaluated for drought response and recovery during consecutive 60‐d drought and 60‐d recovery periods over 2 yr. Using digital image analysis, drought response and recovery were quantified as the number of days to decrease or increase to 50% green ground cover, respectively. Both study years provided unique conditions for investigating drought response as the mean time to reach 50% green ground cover differed by 24 d between the 2 yr of study. Some SA cultivars lost 50% green ground cover in 23 d while other cultivars lasted the entire 60 d drought period without losing 50% green ground cover. Floratam provided the most consistent drought response and recovery compared to other SA cultivars. Once water was no longer limited, cultivars demonstrated up to a 52 d difference in attaining 50% green ground cover. Results could significantly impact home consumer irrigation behaviors and influence consumer expectations of turfgrass following drought conditions.

Selection for freezing tolerance in St. Augustinegrass through somaclonal variation and germplasm evaluation

With 4 figures and 1 table Abstract St. Augustinegrass [Stenotaphrum secundatum (Walt.) Kuntze] is the least cold-hardy turfgrass species. Development of freezing-tolerant St. Augustinegrass cultivars would greatly benefit home owners in many southern states of the US. Towards this breeding goal, 7800 plants regenerated through tissue culture and 36 germplasm accessions were screened for improved freezing tolerance. Among the conditions tested, 1 week at 13°C followed by another week at 3°C, then freezing at −3 to −5°C for 3 h, was found to be suitable to distinguish genotypes in freezing tests. The experiments revealed that germplasm accession Elm4 was significantly more freezing-tolerant under a controlled environment than ‘Raleigh’, the current commercially available, most freezing-tolerant cultivar. In addition, out of 7800 regenerated plants from tissue culture, somaclonal variant SVC3 showed significantly more freezing-tolerant than its parent ‘Raleigh’.

Morphological mutants of St. Augustinegrass induced by gamma ray irradiation

With 3 figures and 3 tables Abstract St. Augustinegrass is a widely used turf and pasture grass in the southern US. ‘Raleigh’ is a cultivar known for superior cold tolerance than other St. Augustinegrass cultivars. However, its coarse-leaf texture and long internodes are undesirable when planted in home lawns. Mutagenesis by gamma ray irradiation was employed to treat node cuttings and calli for inducing semi-dwarf growth phenotype. Dosages of 48.5 and 72.6 Gy were determined as LD50 and LD20 for the cuttings, respectively. Regeneration ability of callus was greatly reduced when irradiated with higher dosages (over 100 Gy). Thirteen morphological mutants were identified among over 3000 node cuttings and 80 pieces of calli treated. Most mutants were semi-dwarf type with reduced internode length and leaf blade length. One mutant had much less and shorter stolons and displayed an upright and tufty growth pattern. The altered morphological traits were stable as shown by their growth performance in various locations and conditions.

Role of Leaf Sheath Lignification and Anatomy in Resistance Against Southern Chinch Bug (Hemiptera: Blissidae) in St. Augustinegrass

Southern chinch bug, Blissus insularis Barber (Hemiptera: Blissidae), is the most serious insect pest of St. Augustinegrass Stenotaphrum secundatum (Walter) Kuntze, a common lawngrass grown in southeastern U.S. states. Host plant resistance to southern chinch bug has been identified in the polyploid St. Augustinegrass 'FX-10' and the diploid 'Captiva'. The objective of this research was to identify possible physical mechanism(s) explaining chinch bug resistance in these cultivars. We studied the distribution of chinch bug salivary sheaths in the preferred tissue for feeding (the axillary shoot) of the two resistant cultivars and two susceptible cultivars, paired for ploidy ('Floratam', polyploid, and Palmetto, diploid). We also investigated the potential role of axillary shoot lignification and anatomy in chinch bug resistance. Salivary sheaths were more abundant on the outermost leaf sheath of axillary shoots of resistant cultivars compared with susceptible cultivars. In contrast, fewer salivary sheaths reached the innermost meristematic tissue in the axillary shoots of resistant St. Augustinegrass cultivars than in the two susceptible cultivars. The polyploid cultivars FX-10 and Floratam had higher total lignin in axillary shoots compared with the diploid cultivars Captiva and Palmetto. However, total lignin content was not correlated with resistance to southern chinch bug. Light microscopic studies found no differences in epidermal layer thickness among resistant and susceptible St. Augustinegrass cultivars. However, transmission electron microscopic studies revealed that the cell walls of the sclerenchyma cells around the vascular bundle of southern chinch bug-resistant FX-10 and Captiva were significantly thicker than the cell walls in susceptible Floratam and Palmetto. Our research suggests that the thick-walled sclerenchyma cells around the vascular bundle play a role in southern chinch bug resistance in St. Augustinegrass, possibly by reducing stylet penetration to the vascular tissue.

St. Augustinegrass Cultivar Influences on Southern Chinch Bug and Predator Populations

Field and greenhouse studies were conducted in Georgia and South Carolina to determine the tolerance and antibiosis effects of 15 St. Augustinegrass genotypes against the southern chinch bug. ‘Floratam’ and ‘Floralawn’ cultivars were highly resistant to southern chinch bug populations in Georgia and South Carolina. When southern chinch bugs were caged on Floratam for 37 days in a greenhouse study, both survivorship and adult reproduction were either stopped or significantly reduced. Color and quality ratings of heavily infested field plantings of ‘Raleigh’ were not significantly reduced even though this cultivar had the highest chinch bug density. Color and quality ratings for ‘Amerishade’ were significantly lower than all other cultivars. The data suggested that the St. Augustinegrass genotypes exhibited different levels of tolerance to infestation by southern chinch bug. The results called into question the established treatment threshold of 20 to 25 chinch bugs per 0.1 m2, which does not consider the tolerance levels among St. Augustinegrass cultivars. Numbers of the predator Lasiochilus palidulus also varied by genotype and were least abundant in the resistant cultivars and most abundant in ‘Winchester.’

The Influence of Silicon on the Components of Resistance to Gray Leaf Spot in St. Augustinegrass

ABSTRACT Soil fertilization with silicon (Si) has been shown to reduce the severity of gray leaf spot, caused by Magnaporthe grisea, in St. Augustinegrass. However, to understand how Si reduces gray leaf spot, it is necessary to study the effects of Si on the components of host resistance. In a greenhouse, six rates (0, 0.5, 1, 2, 5, and 10 t ha−1) of calcium silicate were evaluated for their ability to affect the components of resistance to gray leaf spot in the susceptible (‘Floratam’) and resistant (‘FX-10’) St. Augustinegrass cultivars. Components of resistance assessed were incubation period, latent period, lesion number, lesion area, daily rate of lesion expansion, and number of conidia per lesion. Percent leaf area diseased, a result of the components of resistance, was also evaluated. In this study, the only component of resistance affected by Si was lesion number. Silicon application significantly reduced the number of lesions 26 to 61% when compared to the untreated control in the susceptible cultivar ‘Floratam’. In the resistant cultivar ‘FX-10’, as the rate of calcium silicate increased the number of lesions decreased significantly from 43 to 57%. The percent leaf area diseased in ‘Floratam’ was significantly reduced (18–57%) by increasing rates of silicon when compared to the control. Silicon was the only element to increase significantly in the plant tissue from the amendment of soil with calcium silicate. Integrated disease management strategies for reducing gray leaf spot in both sod production and home lawn settings can be augmented by using Si fertilization to increase plant resistance to gray leaf spot, especially in soils deemed to be low in plant available Si.

Shade Tolerance of St. Augustinegrass Cultivars

The objectives of this research were to rank the relative shade tolerance of some new st. augustinegrass (Stenotaphrum secundatum) cultivars and to determine what levels of shade the various cultivars can tolerate. Two consecutive studies were conducted in a glasshouse at the University of Florida Turfgrass Research Envirotron. Cultivars tested were `Bitter Blue', `Floratam', `Palmetto', `Seville', and `1997-6'. Grasses were grown in full sun or under shade structures that provided 30%, 50%, or 70% shade. In trial 1, `Seville' and `1997-6' generally provided best performance under increasing shade, with worst responses seen in `Floratam'. `Seville' and `1997-6' were predicted to maintain an acceptable quality rating of 6 at all shade levels. In trial 2, `Floratam' again had lowest visual quality scores. At 30% shade, `Seville', `Palmetto', and `Bitter Blue' ranked in the highest category, while only `Seville' and `Bitter Blue' had highest rankings at 50% shade. Reduced density was a major factor in turf decline as shade increased. Most of the cultivars performed best under some degree of shade. With the exception of `Floratam', acceptable visual scores were maintained at shade levels exceeding 60% in trial 1 and up to 61% in trial 2.

Winter Survival of St. Augustinegrass Cultivars

St. Augustinegrass [Stenotaphrum secundatum (Walt.) Kuntze] has low freezing tolerance and suffers winter injury in the southeastern United States. Laboratory methods have determined that the lethal cold temperature of St. Augustinegrass stolons and buds is between -4.5 °C and -7.7 °C. The field survival of St. Augustinegrass to winter freezing is poorly known because most field reports have been based on a single location experiencing a single winter minimum air temperature. The objective of the study was to assess the winter survival of St. Augustinegrass cultivars across a range of winter minimum air temperatures occurring in experimental plantings at 24 Florida counties, following a severe Arctic cold front that moved through Florida beginning 21 Dec. 1989. Except for two counties, the limit for St. Augustinegrass winter survival was a minimum air temperature between -6 °C and -9 °C. Based on a nonlinear estimate using a 3-parameter sigmoidal model (r2 = 0.70, P < 0.0001), 50% survival of St. Augustinegrass would be predicted at -7.9 °C. Time since planting had no relationship with survival. Differences among St. Augustinegrass cultivars were observed at only two counties.