Alan R. Biggs

Intracellular Suberin: Occurrence and Detection in Tree Bark

A protocol is outlined for histochemical detection of intracellular suberin linings on the inner surface of the cell walls in impervious tissues of wounded and infected bark, and in bark forming rhytidome. Thin intracellular suberin linings (circa 0.5 µm) were detected in all 15 woody angiosperms examined. Intracellular suberisation was strongly associated with individual cells or cell layers (boundary zone) that displayed imperviousness with fluid diffusion tests. Tests inc1ude use of phloroglucinol + HCl and Sudan black B to selectively quench autofluorescence of lignin and suberin, respectively. Blue-violet excitation is used to enhance the Sudan IV test for suberin, cutin, and waxes.

Effects of calcium salts on apple bitter rot caused by two Colletotrichum spp.

The effects of three calcium salts on conidial germination, germ tube elongation, growth in vitro, and infectivity in the laboratory and field were studied for the apple bitter rot pathogens, Colletotrichum gloeosporioides and C. acutatum. Calcium chloride, calcium propionate, and calcium silicate at 1,000 μg of calcium per ml had no effect on conidial germination; however, calcium chloride and calcium propionate inhibited germ tube growth by 41 and 50% relative to the control, respectively. All three calcium salts reduced fungal dry weight in liquid culture media. When calcium salt solutions were applied to wounded apples prior to inoculation, fruit treated with calcium chloride and calcium propionate exhibited 30% smaller lesions than those treated with calcium silicate or the control, which were similar. Fruit treated with calcium chloride and calcium propionate exhibited delayed formation of acervuli relative to the control and calcium silicate, which were similar. When fruit were inoculated with varying concentrations of conidia, from 1 × 103 to 1 × 105 conidia per ml, fruit treated with calcium chloride exhibited reduced incidence of infection after inoculations with 1 × 103 conidia per ml. In all tests at 1 ×104 and 1 × 105 conidia per ml, the control and calcium salt treatments resulted in similar incidences of infection after inoculations. In six field trials, where fruit received three weekly dilute applications of calcium solutions, fruit treated with calcium salts and then inoculated with conidia of either C. gloeosporioides or C. acutatum exhibited lower incidences of infection when compared with control fruit. These experiments demonstrate that calcium salts have suppressive activity against the bitter rot pathogens and could be used as part of a disease management program.

Detection of latent infections in apple fruit with paraquat.

Biggs, A. R. 1995. Detection of latent infections in apple fruit with paraquat. Plant Dis. 79:1062-1067. This research was conducted to assess the effect of paraquat on the breakdown of apple fruit (pre- and postharvest), to identify the fungi recovered from paraquat-treated fruit, and to determine the potential use of paraquat in a rapid quantitative measure of the pathogenic component of fruit storability. In 1992, a greater incidence of Golden Delicious and Nittany apple fruit developed acervuli of Colletotrichum acutatum, conidiophores of Alternaria alternata, and pycnidia of Botryosphaeria dothidea after treatment with paraquat than without treatment. In 1993, fungi observed on both cultivars were C. acutatum, B. dothidea, Phoma spp., Phyllosticta solitaria, Penicillium expansum, and A. alternata. Treatment of asymptomatic fruit sections with paraquat facilitated the detection of only B. dothidea, Phoma spp., and P. solitaria on Golden Delicious fruit. Exposure of Nittany fruit to paraquat facilitated the detection of B. dothidea, P. expansum, Phoma spp, and P. solitaria, but not that of C. acutatum or A. alternata. Golden Delicious fruit inoculated with B. dothidea or C. acutatum, harvested when asymptomatic, and treated with paraquat, yielded 80 and 20% infection, respectively, compared with only 6.7 and 0%, respectively, for the untreated controls. Following exposure to paraquat, naturally infected symptomatic Golden Delicious fruit exhibited signs of B. dothidea, P. expansum, A. alternata, Phoma spp., and P. solitaria. The incidence of C. acutatum on paraquat-treated fruit was positively correlated with the incidence after cold storage (r = 0.98) and after cold storage followed by a 4-week incubation at 22 ± 2°C (r = 0.79). The incidence of B. dothidea on paraquat-treated fruit was not correlated with the incidence after cold storage; however, there was a positive correlation after fruit removed from cold storage were incubated at 22°C for 4 weeks (r = 0.95). The incidences of P. expansum and A. alternata after paraquat treatment were correlated with their incidences after only cold storage (r = 0.81 and r = 0.85, respectively). The incidences of Phoma spp. and P. solitaria on paraquat-treated fruit were not correlated with their respective incidences after any storage and incubation conditions.

Anatomical and Physiological Responses of Bark Tissues to Mechanical Injury

Studies of the defense systems of trees have focused mostly on xylem tissues because of their direct economic importance to the forest industry. Because bark tissues shield the xylem from the environment, containment of mechanical injuries and infectious microorganisms by bark tissues is of primary importance. The integrity of normal periderm and the ability of plants to form new periderms at wounds or injuries are essential characteristics for normal plant growth and development. However, in comparison with xylem tissues, responses of periderm and other bark tissues to injury and infection are inadequately defined.

A New View of Sooty Blotch and Flyspeck

Sooty blotch and flyspeck (SBFS) fungi colonize the surface wax layer of the fruit of apple, pear, persimmon, banana, orange, papaya, and several other cultivated tree and vine crops. In addition to colonizing cultivated fruit crops, SBFS fungi also grow on the surfaces of stems, twigs, leaves, and fruit of a wide range of wild plants. The disease occurs worldwide in regions with moist growing seasons. SBFS is regarded as a serious disease by fruit growers and plant pathologists because it can cause substantial economic damage. The smudges and stipples of SBFS often result in downgrading of fruit from premium fresh-market grade to processing use. This review describes the major shifts that have occurred during the past decade in understanding the genetic diversity of the SBFS complex, clarifying its biogeography and environmental biology, and developing improved management strategies.

Pathogenic diversity of Phytophthora sojae and breeding strategies to develop Phytophthora-resistant soybeans

Phytophthora stem and root rot, caused by Phytophthora sojae, is one of the most destructive diseases of soybean [Glycine max (L.) Merr.], and the incidence of this disease has been increasing in several soybean-producing areas around the world. This presents serious limitations for soybean production, with yield losses from 4 to 100%. The most effective method to reduce damage would be to grow Phytophthora-resistant soybean cultivars, and two types of host resistance have been described. Race-specific resistance conditioned by single dominant Rps (“resistance to Phytophthora sojae”) genes and quantitatively inherited partial resistance conferred by multiple genes could both provide protection from the pathogen. Molecular markers linked to Rps genes or quantitative trait loci (QTLs) underlying partial resistance have been identified on several molecular linkage groups corresponding to chromosomes. These markers can be used to screen for Phytophthora-resistant plants rapidly and efficiently, and to combine multiple resistance genes in the same background. This paper reviews what is currently known about pathogenic races of P. sojae in the USA and Japan, selection of sources of Rps genes or minor genes providing partial resistance, and the current state and future scope of breeding Phytophthora-resistant soybean cultivars.

Relative Susceptibility of Selected Apple Cultivars to Colletotrichum acutatum

Eighteen apple cultivars were tested in the field and laboratory for their relative susceptibility to one of the bitter rot pathogens, Colletotrichum acutatum. Fruit were inoculated in the field at 3 to 4 weeks preharvest with cheesecloth strips soaked in a conidia suspension. In the laboratory, detached fruit were inoculated using a conidia suspension in capped, sterile microcentrifuge tubes attached to the fruit surface with modeling clay. The same fruit as above also were inoculated over a wound on the side opposite the nonwounded inoculation. Fruit were tested for relative susceptibility to the fungus with five criteria: disease incidence and severity of attached fruit in the field, disease incidence and severity of detached fruit in laboratory inoculations of nonwounded fruit, and disease severity in laboratory inoculations of wounded fruit. Relative cultivar ranks from field tests were not reproducible in the 2 years studied, whereas laboratory tests showed moderate reproducibility with nonparametric rank correlation tests. Based on the laboratory data from 2 years of study, cultivars were classified into four relative-susceptibility groups: most susceptible: Pristine, Honeycrisp, and Ginger Gold; highly susceptible Yataka, Sansa, Arlet, and Enterprise; moderately susceptible: Sunrise, Golden Supreme, PioneerMac, GoldRush, Golden Delicious, and Creston; and least susceptible: Fuji. Compared to previous cultivar rankings, the results of the present study indicate that new apple cultivars from the first NE-183 planting show no improvement in resistance to C. acutatum.

Select Calcium Compounds Reduce the Severity of Phytophthora Stem Rot of Soybean

This study investigated the effects of several calcium compounds on Phytophthora stem rot of soybean (Glycine max) and fungal growth and zoospore release of a Phytophthora sojae isolate in vitro. All concentrations of five formulated calcium products [Ca(COOH)2-A, Ca(COOH)2-B, Ca(COOH)2-C, CaSO4-A, and CaCl2-A] and two chemical compounds [CaCl2 and Ca(NO3)2] applied prior to inoculation significantly suppressed disease incidence. Among all the products and chemicals, Ca(COOH)2-A was the most effective in suppressing the incidence of disease. In most cases, no significant relationship was observed between inhibition of growth rate in vitro and disease reduction in growth chamber tests. Therefore, disease suppression recorded in laboratory experiments using pathogen mycelium was likely due to the responses of plant tissues rather than the direct inhibition of pathogen fungal growth by the calcium compounds. The extent of disease reduction was related to increased calcium uptake by plants, suggesting that calcium was the effective element in reducing Phytophthora stem rot. Seedling tray experiments using zoospores indicated that the application of 10 mM Ca(COOH)2-A was more effective for reducing incidence of disease under growth chamber conditions, compared to other concentrations. The presence of 4 to 20 mM of all seven calcium solutions decreased the release of zoospores, although 0.4 mM of all compounds significantly increased zoospore release. Therefore, disease reduction in the growth-chamber experiments was due to the multiple effects of direct suppression on zoospore release and fungal growth in combination with the response of the host plant tissue to Ca(COOH)2-A.

Field application of calcium to reduce Phytophthora stem rot of soybean, and calcium distribution in plants.

The effect of calcium compounds [Ca(HCOO)2-A and Ca(NO3)2] on the incidence of Phytophthora stem rot of soybean (Glycine max) cv. Tanbakuro was investigated in the field. Disease incidence in control plants in three fields naturally infested with Phytophthora sojae ranged from 11.7 to 52.0% at 140 days after transplanting. Independent of the pathotype diversity, 4 and 10 mM of the calcium compounds applied twice (prior to transplanting and 14 days after transplanting) significantly suppressed disease incidence and delayed onset. Ca(HCOO)2-A (Suicaru) was more effective than calcium nitrate for reducing disease incidence. In most cases, the calcium amendments increased plant height, number of nodes and pods, and seed yields, and reduced low-quality seeds. Scanning electron microscopy with fresh samples showed increased accumulation of calcium crystals around the cambium and xylem elements of soybean plants treated with 10-mM Ca(HCOO)2-A and Ca(NO3)2. Mycelial penetration was inhibited at these sites. These results indicated that calcium-rich areas may be more resistant to invasion by P. sojae, and the calcium crystals may play an important role in calcium ion storage and its availability for those tissues to maintain long-term field resistance.

Effect of Inoculum Concentration and Calcium Salts on Infection of Apple Fruit by Botryosphaeria dothidea

The objective of this study was to determine the effects of calcium salts on growth of Botryosphaeria dothidea and incidence of white rot. The relative virulence of five B. dothidea isolates was determined using the apple cultivars Fuji, Gala, Golden Delicious, Liberty, and York. Cultivar and isolate differences in lesion diameter were significant. Cultivar differences occurred between Fuji, which was most susceptible, and Gala, which was least susceptible. Isolates PA-1 and PA-2 were most virulent. Isolate PA-4 was used at conidia concentrations of 1 × 104, 1 × 105, 1 × 106, and 1 × 107 conidia per ml on Golden Delicious fruit to test the effects of four inoculum concentrations on fruit infection by B. dothidea. Incidence of infection on detached, nonwounded fruit increased as inoculum concentration increased. The effects of four calcium salts on infection of wounded fruit by conidia were examined in field and laboratory experiments. In the field experiments, lesion size was slightly reduced on fruit treated with either calcium hydroxide or calcium silicate after wounding and prior to inoculation with conidia of B. dothidea, whereas lesions that developed at wounds treated with calcium chloride or calcium propionate were similar to those of the control. In the laboratory experiments, in several instances supplementation with calcium salts resulted in increased lesion diameter relative to the control. Two of five isolates tested showed increased percent germination with all four calcium salts, and one isolate showed increased germination in the presence of two of the four calcium salts. Germ tube elongation was not affected by calcium salts for four out of five isolates tested. There was significant variation among isolates and calcium salts on the growth of B. dothidea in liquid culture. Calcium chloride and calcium hydroxide enhanced the growth of three isolates and had no effect on the other three isolates. Calcium propionate inhibited the growth of four isolates and had no effect on the other two isolates. Calcium silicate inhibited the growth of one isolate and had no effect on the other five isolates.