Seeds as Vehicles for Pathogen Importation

Since the 1900s, consumer demand for new plant products gave opportunity for many plant pathogens to disseminate to new areas on imported seeds. New markets for plant commodities encouraged plant breeders to begin collecting seed stocks from abroad. The birth of new seed companies extend their markets to new area. These events began the global dissemination of many seedborne pathogens. Many seedborne pathogens gained entry and escaped detection by specific traits that favored their dissemination. Three recent case scenarios are presented that illustrate how plant pathogens that passively employ the seed coats of their host achieved global dissemination and permanence in each patho-system. Evidence is presented to show that asparagus (Asparagus officinalis) seed produced in the US acted as a vehicle for disseminating one vegetatively compatible group (VCG) of a pathogenic fungus on asparagus called Fusarium proliferatum throughout new plantings in Australia. Similarly, public demand for Mediterranean cuisine in the US and abroad during the last 20 years led to an increase in the importation of basil (Ocimum basilicum) seed along with an inconspicuous fungus called Fusarium oxysporum. The fungus caused a destructive disease called Fusarium wilt of basil that appeared in over 25 separate locals spanning three continents. The third example demonstrated how new developments in lupine (Lupinus spp.) cultivars and increased public demand led to the global dispersal of a seedborne pathogen called Colletotrichum gloeosporioides. Each case highlights how these pathogens use seeds, humans, and particular traits to disperse globally in short period of time.     

Gas Exchange and Emission of Chlorophyll Fluorescence during the Monocycle of Rust, Angular Leaf Spot and Anthracnose on Bean Leaves as a Function of their Trophic Characteristics

Measurements related to gas exchange and chlorophyll fluorescence emission were taken from healthy and diseased bean leaves with rust, angular leaf spot, and anthracnose during lesion development for each disease. The experiments were performed at different temperatures of plant incubation, and using two bean cultivars. The main effect of temperature of plant incubation was in disease development. There was no significant difference between cultivars in relation to disease development and in magnitude of physiological alterations when disease severity was the same for each cultivar. These diseases reduced the net photosynthetic rate and increased the dark respiration of infected leaves after the appearance of visible symptoms and the differences between healthy and diseased leaves increased with disease development. The transpiration rate and stomatal conductance were stable during the monocycle of rust, however, these two variables decreased in leaves with angular leaf spot and anthracnose beginning with symptom appearance and continuing until lesion development was complete. Carboxylation resistance was probably the main factor related to reduction of photosynthetic rate of the apparently healthy area of leaves with rust and angular leaf spot. Reduction of the intercellular concentration of CO₂, due to higher stomatal resistance, was probably the main factor for leaves with anthracnose. Chlorophyll fluorescence assessments suggested that there was no change in electron transport capacity and generation of ATP and NADPH in apparently healthy areas of diseased leaves, but decreases in chlorophyll fluorescence emission occurred on visibly lesioned areas for all diseases. Minimal fluorescence was remarkably reduced in leaves with angular leaf spot. Maximal fluorescence and optimal quantum yield of photosystem II of leaves were reduced for all three diseases. Bean rust, caused by a biotrophic pathogen, induced less damage to the regulation mechanisms of the physiological processes of the remaining green area of diseased leaves than did bean angular leaf spot or anthracnose, caused by hemibiotrophic pathogens. The magnitude of photosynthesis reduction can be related to the host–pathogen trophic relationships.     

Effects of Nutrition on Desiccation Tolerance and Virulence of Colletotrichum truncatum and Alternaria alternata Conidia

The promising mycoherbicides Colletotrichum truncatum and Alternaria alternata were grown respectively in liquid and solid semi-defined media. C. truncatum conidia produced in a medium with a C:N ratio of 5:1 showed higher desiccation tolerance (survival during storage) at 15% relative humidity and 25°C, greater germination on the host leaf and greater disease expression on Sesbania exaltata than those produced in media with C:N ratios of 15:1 or 40:1. Similar results were obtained with conidia of A. alternata produced on a medium with a C:N ratio of 15:1. Conidia washed with 0.9% (w/v) NaCl produced higher tolerance to desiccation, and greater disease incitement, than unwashed conidia of C. truncatum or conidia washed with water. In contrast, washing had no positive effect on desiccation tolerance in A. alternata .     

Mycelial elongation and sporulation of two fungi on amended media in light or dark

Botrytis allii andCollectotrichum dematium are onion pathogens which can infect in the field and cause decay in storage. Some phenolics can hinder development of these fungi, but the effect of cytokinins is not clear. Cytokinins (kinetin or 6-benzyladenine) or phenolics (caffeic or chlorogenic acids) were added to agar at concentrations of 0 to 10^–3 M. Cultures were continuously irradiated with fluorescent light or maintained in the dark for 6 days. On unamended media, final mycelial elongation was 45 or 17.8 mm and sporulation was 28 or 10.6 × 10⁴ spores/ml forBotrytis andColletotrichum, respectively. ForBotrytis, mycelial elongation was slightly (5%) but significantly increased and sporulation increased by 21% by incubation on phenolics as compared to cytokinins. Mycelial extension ofColletotrichum was not affected by amendment. Sporulation ofColletotrichum on kinetin was 16 to 28% greater than on the other amendments. As amendments concentration increased elongation of mycelia of both fungi decreased. Sporulation ofBotrytis increased by 60% as amendment concentration increased from 0 to 10^–5 M and then decreased 25% at 10^–3 M. As amendment concentration increased from 0 to 10^–3 M, sporulation ofColletotrichum increased by 45%. Incubation in light increased mycelial extension 3 to 17% forBotrytis andColletotrichum respectively, and sporulation was increased approximately 78% for both fungi. These compounds do not appear to inhibit development of theseBotrytis orColletotrichum species in culture.     

Role of foliar hemibiotrophic and fruit resistance in anthracnose‐resistant strawberry genotypes for annual hill plasticulture systems

Anthracnose fruit rot (AFR) caused by Colletotrichum acutatum is one of the most devastating diseases of strawberry (Fragaria × ananassa) in plasticulture systems in the Southeast US. Host resistance offers the best option to limit crop losses in both nursery and fruiting fields. To evaluate levels of anthracnose resistance and elucidate the role of foliar and fruit resistance in overall field AFR resistance, we tested 14 strawberry genotypes including numerous selections from the North Carolina State University strawberry breeding programme. Inoculations of plug plants with three representative C. acutatum isolates prior to field‐set indicated that the commercially standard cultivar Chandler was highly susceptible, with an average fruit rot incidence of over 72% the following spring. In contrast, breeding lines such as NC C99‐13 and NC C02‐63 showed superior resistance with AFR incidence values of 23.6% and 11.1%, respectively, and showed superior marketable yields. An average hemibiotrophic infection (HI) severity on foliage (percent leaf area covered with acervular growth after paraquat treatment and incubation) did not correlate (r = 0.57) well with in vitro AFR severity on detached fruit, indicating different mechanisms may be operative for resistance to foliar HI and fruit rot resistance. Multiple regression analysis indicated that in vitro fruit rot resistance expressed by lesion diameter and severity of foliar hemibiotrophic infections may be utilized to predict field AFR incidence. Strawberry genotypes bred for resistance against both fruit rot and foliar HI could be effectively selected by using rank‐sum classification methods and this process offers an effective strategy to advance selections for superior AFR field resistance.     

Der mit der Versuchsdauer anwachsende Resistenzfaktor

Abstract

Pseudomonas spp. strains capable of inducing systemic resistance were applied to sugarcane by sett treatment followed by soil applications in the field. Later the fungal pathogen Colletotrichum falcatum causing red rot disease of sugarcane was inoculated in the treated canes and its colonization was assessed by ELISA at different nodal positions above the point of inoculation. Studies with three cvs showed a significant variation in pathogen colonization only in disease susceptible cv CoC 671 and not with cvs Co 8021 and BO 91, moderately susceptible and moderately resistant to the disease, respectively. In further studies when pathogen colonization was assessed on the entire stalks of cv CoC 671, the pathogen titre was significantly reduced from three nodes upwards in the treated canes. In the upper nodes no pathogen colonization was noticed in bacteria-treated canes, whereas in the control all the nodes recorded higher titre for pathogen infection. Incorporation of chitin in the talc formulation caused further reduction in fungal colonization in the stalks.     

Reaction of Some Coffea arabica Genotypes to Strains of Colletotrichum kahawae, the Cause of Coffee Berry Disease

Pathogenicity tests were performed on 11 genotypes of Coffea arabica using single‐isolate suspensions of Colletotrichum Kahawae obtained from 90 monoconidial isolates. The objective of this study was to estimate the proportion of pathogenic variation corresponding 10 differences in aggressiveness and virulence (races). A large part of the variation in the pathogen population was due to aggressiveness. The differential effects were too small to suggest conclusively that races exist. This paper discusses the possible causes for the observed small differential interaction and suggests breeding strategies that not only prevent possible adaptation of the pathogen to resistant varieties but also limit variation for resistance due to differences in aggressiveness of the pathogen.     

Bioherbicidal enhancement and host range expansion of a mycoherbicidal fungus via formulation approaches

Eastern black nightshade (Solanum ptycanthum; EBN) is a problematic weed partly due to its tolerance or resistance to certain herbicides. We examined the effects of an invert emulsion (IE) on the host range and weed control efficacy of the fungus Colletotrichum coccodes (NRRL strain 15547) for biocontrol of EBN. Greenhouse tests demonstrated that several other solanaceous weeds were also infected and killed, and field tests revealed >90% EBN control and dry weight reduction in plants treated with the fungus-IE formulation. These results demonstrate that this IE formulation can promote the efficacy of this bioherbicidal pathogen.     

Hydroponic Seedling Bioassay for the Bioherbicides Colletotrichum truncatum and Alternaria cassiae

A rapid bioassay was developed to measure the bioherbicidal efficacy of spore preparations of the pathogens Colletotrichum truncatum (Schwein.) Andrus and W. D. Moore and Alternaria cassiae Jurair and Khan on hemp sesbania (Sesbania exaltata) and sicklepod (Cassia obtusifolia), respectively. The system uses 4-day-old dark-grown seedlings (grown hydroponically in paper towel cylinders) which were sprayed with spore suspensions. Shoot lengths were monitored non-destructively, and recorded over time under conditions of dark growth, 90-100% relative humidity and 25 C. Shoot growth inhibition and stem collapse (mortality) were directly related to the spore concentration applied. Generally, at 10 3 - 10 4 spores ml-1, these pathogens caused significant shoot growth inhibition within 25-30 h and seedling death within 40-50 h. This bioassay has been used to study herbicide-pathogen interactions, and may be extended to determine the bioherbicidal efficacy of different pathogen isolates, pathovars or spore formulations. This technique is more rapid, uses a lower inoculum volume, requires less space and is performed under more controlled conditions than conventional greenhouse bioassay methods. The data obtained are more quantitative than those obtained from bioassays relying on visual rating systems.     

浙江无核柿炭疽病菌鉴定及附着胞形成过程中的核相变化

浙江无核柿炭疽病近年来在浙江淳安地区严重发生,根据形态学特征病原菌鉴定为胶孢炭疽菌ColletotrichumgloeosporioidesPenz.,在枝条病斑上的分生孢子盘通常不产生刚毛,分生孢子顶端顶部钝圆,基部平截,分生孢子盘中的孢子包埋在基质中,紧密结合在一起。分生孢子在自然寄主和人工培养条件下形态特征相似。6个柿树炭疽菌菌株的rDNAITS序列联配显示,其序列是相同的。用UPGMA方法分析ITS1-ITS2序列构建的炭疽菌系统发育树把6个柿树炭疽菌菌株和其它寄主上的胶孢炭疽菌或其有性型围小丛壳菌菌系分入同一个组,与根据形态学的鉴定结果一致。在附着胞形成过程中,用DAPI荧光染色观察到核相发生两次有丝分裂变化。第一次有丝分裂发生在分生孢子固着聚苯乙烯塑料培养皿3-4h后,随后,分生孢子中部形成一个隔膜,把它分成两个细胞;6~7h后,分生孢子发生第二次有丝分裂。分裂后,一个核通过芽管移入附着胞中。