Brown spot disease of citrus caused by Phaeoisariopsis sp.

Brown spot disease of Citrus spp. was shown to be caused by Phaeoisariopsis sp. It severely affects all varieties of sweet orange, tangerine, grapefruit, lime and rough lemon, smooth lemon being relatively resistant. Fruits and leaves are much more susceptible than stems on which symptoms are rare. On the young fruit circular, slightly sunken, brown necrotic lesions are often associated with a surrounding ring of raised epicarp, giving the fruit a blistered appearance. Lesions on older fruits are usually flat and brown and surrounded by yellow haloes. Premature abscission, especially of young fruits, is common. Leaf symptoms start as greenish yellow patches and a fully formed leaf spot consists of light brown or greyish centre which is surrounded by a dark brown margin bordered by a yellow halo. Generalised foliar chlorosis, caused by coalescence of several lesions, culminates in premature defoliation. During wet weather, centres of both fruit and foliar lesions sporulate and become black. Stem lesions, dark brown and mostly occurring as extensions of petiole lesions, may coalesce causing stem die-back or resulting in the formation of corky internodal lesions. In artificial culture, the fungus sporulated only on media containing an extract of citrus leaf or fruit peel. The characteristics of the fungus mostly based on examination of structures formed on naturally infected parts of the plant, are described. This is probably the first report of a Phaeoisariopsis sp. on citrus.     

Scab disease of cowpea (Vigna unguiculata) caused by Sphaceloma a species of the fungus

Scab disease of cowpea (Vigna unguiculuta) was shown to be caused by Sphaceloma sp. It affects all above ground parts of the plant. The first symptom of the disease, appearing within 3 to 6 days of inoculation, is puckering of the lamina. Spots on mature leaves are white with or without brown margins. Typical scab lesions on petiole, stem, peduncle and pod are white turning dark brown when chlamydospores are formed and are oblong-elongate except for pod lesions that are ovoid. The most destructive phase is scab of the flowering axis which causes flower and, or, pod abortion or completely prevents flower formation. Inoculation of asparagus pea (V. sesquipedalis) with a cowpea isolate of Sphaceloma sp. produced symptoms similar to those on cowpea. Inoculated hyacinth bean (Lablab niger) produced atypical mild lesions. The following legumes were not affected when artificially inoculated with the fungus: black gram (Phaseolus mungo), green gram (P, aureus), French bean (P. vulgaris), Lima bean (P. lunatus), groundnut (Arachis hypogaea), and soyabean (Glycine max). The characteristics of the fungus on potato dextrose agar are described.     

Diagnosis and management of Sclerotinia stem rot (white mould) of lentils in Greece

The paper describes the field-level symptoms, the identification and management of Sclerotinia stem rot of lentil caused by the soilborne plant pathogen Sclerotinia sclerotiorum in Greece. Regarding symptoms at a field level, initially plants before flowering turn yellow with roots and the base of the plants become brown; then rotten plants exhibit a dry stem and die. On the diseased tissue, at the base of the stem, the typical white mycelium and the resting bodies (sclerotia) were observed. According to our pathogenicity studies in vitro, on the infected plant tissues the fungus first develop a characteristic fluffy white mycelium which will give rise to large black sclerotia, the most obvious evidence of plants infected with S. sclerotiorum. Finally, concerning evaluation of fungicides, isolates of S. sclerotiorum were sensitive to thiophanate-methyl and to triazole fungicides. Thiophanate-methyl and triazole fungicides proved to be most effective in controlling the disease emerged from mycelium or sclerotia.     

The role of the whitefly,Bemisia tabaci (Gennadius), and farmer practices in the spread of cassava brown streak ipomoviruses

Cassava brown streak disease (CBSD) is arguably the most dangerous current threat to cassava, which is Africa's most important food security crop. CBSD is caused by two RNA viruses: Cassava brown streak virus (CBSV) and Ugandan cassava brown streak virus (UCBSV). The roles of the whitefly Bemisia tabaci (Gennadius) and farmer practices in the spread of CBSD were investigated in a set of field and laboratory experiments. The virus was acquired and transmitted by B. tabaci within a short time (5–10 min each for virus acquisition and inoculation), and was retained for up to 48 hr. Highest virus transmission (60%) was achieved using 20–25 suspected viruliferous whiteflies per plant that were given acquisition and inoculation periods of 24 and 48 hr, respectively. Experiments mimicking the agronomic practices of cassava leaf picking or the use of contaminated tools for making cassava stem cuttings did not show the transmission of CBSV or UCBSV. Screenhouse and field experiments in Tanzania showed that the spread of CBSD next to spreader rows was high, and that the rate of spread decreased with increasing distance from the source of inoculum. The disease spread in the field up to a maximum of 17 m in a cropping season. These results collectively confirm that CBSV and UCBSV are transmitted by B. tabaci semipersistently, but for only short distances in the field. This implies that spread over longer distances is due to movements of infected stem cuttings used for planting material. These findings have important implications for developing appropriate management strategies for CBSD.     

Pythium vexans Causing Stem Rot of Dendrobium in Yunnan Province,China

Since 2002 a severe root and stem disease of Dendrobium has occurred periodically each year in the plantations of Simao City, Yunnan Province, China. Symptoms included water‐soaked and brown lesions, and rot of tissues. Based on the morphological characteristics and the internal transcribed spacer‐1, 5.8S ribosomal RNA gene, and internal transcribed spacer 2 and β‐tubulin gene sequences, the pathogen was identified as Pythium vexans de Bary. The pathogenicity of the fungus was confirmed by satisfying Koch’s postulates. This is the first world record of stem rot of Dendrobium caused by P. vexans.     

Phytophthora-Stammbasisfäule an Dieffenbachia maculate (Erreger: P. mexicana Hots. et Hart.)

Stem base rot of Dieffenbachia maculata caused by Phytophthora mexicana During the last years a stem rot of Dieffenbachia maculata (Lodd.) G. Don has been observed in greenhouses of several commercial nurseries, located in different parts of the Federal Republic of Germany. The disease partly caused heavy losses. From all diseased plant parts (stem base, petiole and root) the same fungus could be isolated. It was diagnosed as Phytophthora mexicana Hots, et Hart., which is closely related to P. palmivora (Butl.) Butl. Apparently the former species is recorded for the first time as the causal organism of a Dieffenbachia stem rot and also for the first time in Europe. It is supposed that the fungus was imported to Germany via the Netherlands because all plants had been bought there, which diseased later.     

Occurrence of Grey Leaf Spot of Sedum erythrostictum Caused by Cercospora cf. pseudokalanchoes in China

Sedum erythrostictum is a perennial herb in the Crassulaceae family, which is a traditional Chinese medicine used for the treatment of hepatitis, dysentery, herpes zoster and swellings. In 2014, a grey leaf spot disease causing significant damage to plants of S. erythrostictum occurred in the Medicinal Herb Garden of Jilin Agricultural University, Jilin Province, China. The fungus mainly infected the leaves. The necrotic lesions on the leaves were circular or elliptical, amphigenous, greyish brown to brown, slightly concave and surrounded by a dark brown distinct margin. The causal agent from symptomatic tissues was identified as Cercospora cf. pseudokalanchoes based on the symptoms, morphological characteristics, molecular identifications and pathogenicity tests. To our knowledge, this is the first formal report of grey leaf spot of S. erythrostictum caused by C. cf. pseudokalanchoes in China.     

Pathogenicity of Epicoccum sorghinum towards dragon fruits (Hylocereus species) and in vitro evaluation of chemicals with antifungal activity

Recent reports of plant diseases that result in yield reduction and increasing demand for dragon fruits raise concerns of fruit supply shortage. Emerging plant diseases may play an important role in increasing yield losses and reducing the availability of stem cuttings (source of planting materials). Understanding the aetiology of current and new diseases of dragon fruit is important to address production issues and to formulate effective disease control measures. This study reports Epicoccum sorghinum as a potential emerging pathogen of dragon fruit. Epicoccum sorghinum MBDF0024a was isolated from dragon fruit stems (Hylocereus monacanthus) showing brown spot symptoms. DNA sequence of the internal transcribed spacer (ITS-rDNA), beta tubulin and actin gene regions of fungal isolate MBDF0024a had high similarities to E. sorghinum stains. Epicoccum sorghinum MBDF0024a was pathogenic to three cultivated dragon fruit species (Hylocereus undatus, H. monacanthus and H. megalanthus) in repeated laboratory and glasshouse trials. Large brown lesions developed on 3-week-old inoculated rooted stem cuttings 3 days postinoculation (dpi). Yellowing of the lesion (advance part) started at five dpi, and at seven dpi, yellowing was observed in the stem. As there are no reported control measures for diseases caused by E. sorghinum, this study screened chemicals with antifungal properties. A biopesticide containing B. subtilis (2 ml/400 ml), and chemicals isoprothiolane (2.25 ml/400 ml), mancozeb (2 g/400 ml) and pyraclostrobin (1 ml/400 ml) (chemical control) completely inhibited the in vitro growth of E. sorghinum MBDF0024a. The results establish E. sorghinum as a new and emerging pathogen of dragon fruit that could be a major yield-limiting disease if left uncontrolled. The biopesticide can be considered a fairly safe option for disease management, but glasshouse and field studies are needed for validation.     

Bacterial soft rot of tomato in plastic greenhouses in Crete

During recent years a new disease has been noticed on tomatoes grown in Polythene greenhouses in Crete. Early symptoms are yellowing of the lower leaves, and a yellow brown discoloration of the pith and stem xylem. As leaves wilt and die there is progressive yellowing towards the top of the plants. A progressive disintegration of the cortical tissues follows which results in a soft rot and a longitudinal splitting of the stem running mainly upwards. Soft rot of the fruits rarely appears. Severely infected plants may wilt and die, but other less affected plants often survive and yield normally. Very vigorous plants grown under humid conditions are more susceptible. Often more than 20% of the plants are infected. Isolations were made from stem (xylem, cortex and pith), from leaf xylem and from fruits of infected tomato plants collected throughout the island from 1979 to 1985. Bacteria of the genus Erwinia and Pseudomonas were consistently isolated. On the basis of physiological and biochemical characters of 49 representative pathogenic isolates, 22 were identified as Erwinia carotovora subsp. carotovora, 10 as Erwinia carotovora subsp. atroseptica, four as Pseudomonas viridiflava and 13 as Pseudomonas fluorescens biotype I. All disease symptoms were reproduced when artificial inoculations were made with the above isolates in the laboratory (20°C and 100% r.h.) on 3–4 week tomato plants and in a commercial greenhouse on 4–5 months tomato plants. Bacteria used for inoculations were reisolated. Results indicated that the disease symptoms as described may be caused by four different bacteria species.     

Symptomology and etiology of a new disease,yellow stunt,and root rot of standing milkvetch caused by <Emphasis Type="Italic">Embellisia</Emphasis> sp. in Northern China

An Embellisia sp. has been established as the cause of a new disease of the herbaceous perennial forage legume, ‹standing milkvetch’ (Astragalus adsurgens Pall.) in Northern China, which severely reduces plant density and degrades A. adsurgens stands. The disease was common at an experimental location in Gansu Province where it was recognized by the occurrence of stunted plants with reddish-brown stems and yellow and necrotic leaf blades. An Embellisia sp. was isolated from symptomatic stem, leaf blade, petiole, and root tissues at varying frequencies of up to 90%. Single-spore isolates grew very slowly on PCA, PDA, V-8 and, wheat hay decoction agar. Pathogenicity was confirmed by inoculation of seeds, dipping 2-day-old pre-germinated seedlings in inoculum and spraying inoculum on 6-month-old plants. Symptoms on test plants included yellow leaf lesions, brown lesions on stems and petioles, stunted side-shoots with yellow, small, distorted and necrotic leaves, shoot blight, bud death, crown rot, root rot, and plant death. The disease is named as ‹yellow stunt and root rot’ of A. adsurgens to distinguish it from diseases caused by other known pathogens. Embellisia sp. is also pathogenic to A. sinicus but not to 11 other tested plant species.     

Characterization of a group I intron in the nuclera rDNA differentiatingPhialophora gregata f. sp.adzukicola fromP. gregata f. sp.sojae

An insertion sequence was detected near the 3′ end of the nuclear small subunit rDNA in isolates ofPhialophora gregata f. sp.adzukicola, the causal agent of the brown stem rot disease of adzuki bean. This insertion sequence was absent in isolates ofP. gregata, f. sp.sojae which causes brown stem rot of soybean. The insertion sequence is 304 bp long and contains all the characteristics of group I introns. These characteristics include, the four conserved sequence elements (P, Q, R, and S), a U at the 5′ splice site of the exon, a G at the 3′ splice site of the intron, a putative internal guiding sequences; the sequence also fits a secondary structure model for group I introns. Similar to most group I introns found in nuclear small subunit rDNA, the intron was located in a highly conserved region and is devoid of long open reading frames. This intron provides a convenient marker for use in conventional PCR to separateP. gregata f. sp.adzukicola fromP. gregata f. sp.sojae.     

Cassava brown streak virus Ham1 protein hydrolyses mutagenic nucleotides and is a necrosis determinant

Cassava brown streak disease (CBSD) is a leading cause of cassava losses in East and Central Africa, and is currently having a severe impact on food security. The disease is caused by two viruses within the Potyviridae family: Cassava brown streak virus (CBSV) and Ugandan cassava brown streak virus (UCBSV), which both encode atypical Ham1 proteins with highly conserved inosine triphosphate (ITP) pyrophosphohydrolase (ITPase) domains. ITPase proteins are widely encoded by plant, animal, and archaea. They selectively hydrolyse mutagenic nucleotide triphosphates to prevent their incorporation into nucleic acid and thereby function to reduce mutation rates. It has previously been hypothesized that U/CBSVs encode Ham1 proteins with ITPase activity to reduce viral mutation rates during infection. In this study, we investigate the potential roles of U/CBSV Ham1 proteins. We show that both CBSV and UCBSV Ham1 proteins have ITPase activities through in vitro enzyme assays. Deep-sequencing experiments found no evidence of the U/CBSV Ham1 proteins providing mutagenic protection during infections of Nicotiana hosts. Manipulations of the CBSV_Tanza infectious clone were performed, including a Ham1 deletion, ITPase point mutations, and UCBSV Ham1 chimera. Unlike severely necrotic wild-type CBSV_Tanza infections, infections of Nicotiana benthamiana with the manipulated CBSV infectious clones do not develop necrosis, indicating that that the CBSV Ham1 is a necrosis determinant. We propose that the presence of U/CBSV Ham1 proteins with highly conserved ITPase motifs indicates that they serve highly selectable functions during infections of cassava and may represent a euphorbia host adaptation that could be targeted in antiviral strategies.     

Morphological and molecular identification of Colletotrichum siamense,a novel leaf pathogen associated with Sterculia lanceolata recorded in China

Sterculia lanceolata, an important tropical woody plant, has high ornamental and medicinal value. To our knowledge, only brown root disease in this plant has been reported. In Nanning, Guangxi, China, an outbreak of leaf blight disease was observed on S. lanceolata in June 2019, with the leaf infection rate ranging from 80% to 100%. The disease seriously affected the leaves of trees and caused economic loss. Eight isolates were recovered from the infected leaves of different trees, and the pathogenicity was then determined by the methods of mycelial disc and conidial suspension, fulfilling Koch's postulates. According to the morphological and molecular biological characteristics of isolates, the pathogen causing leaf blight on S. lanceolata was identified as Colletotrichum siamense. Accurate identification of the pathogen provides a reliable basis for the control of the disease.     

First report of leaf spot disease caused by Phoma eupyrena Sacc. on Aloe vera from Madhya Pradesh,India

A leaf spot disease caused by Phoma eupyrena Sacc. on Aloe vera was reported from Madhya Pradesh, India. Disease symptoms were observed as irregular to elongated, sunken lesion on both leaf surfaces which later turns creamish brown with maroon margin. The fungal colonies were whitish or light pale to dark grey colour. The pycnidia were glabrous and solitary, papillate, and indistinctly uniostiolate averages 100–260 μm in diameter. Conidia were ellipsoidal, 4.5–7.5 × 2.5–3.75 μm in size, with two large guttules. To the best of our knowledge, this is the first report of P. eupyrena causing leaf spot disease on A. vera in India.     

Etiology of a Root Rot Disease Complex of Alstroemeria in Alberta, Canada

Fusarium oxysporum, Pythiu-m ultimum, and Rhizoctonia solani were isolated from the basal stems of diseased alstroemeria showing symptoms of dark brown stripes along leaf margins, leaf chlorosis, plant wilting, browning or rotting of basal stem, rhizome, and storage and fibrous roots. The pathogen isolated most frequently was Fusarium spp. (40.5 % of plants examined). Pythium spp. and R. solani were isolated less frequently (5.5 % and 6.8 % of plants examined, respectively). F. oxysporum caused the highest mortality in alstroemeria when rhizomes were grown in unsterilized soil-less mix medium. This is the first report in North America of a root-rot disease complex affecting alstroemeria.     

Quantification and characterisation of <Emphasis Type="Italic">Trichoderma</Emphasis> spp. from different ecosystems

Basal stem rot of oil palm caused by Ganoderma boninense is of major economic importance. Observations of the low incidence of disease due to Ganoderma species in natural stands, suggest that the disease is kept under control by some biological means. Trichoderma spp. are saprophytic fungi with high antagonistic activities against soil-borne pathogens. However, their abundance and distribution are soil and crop specific. Trichoderma species have been found to be concentrated in the A1 (0–30 cm) and Be soil horizons (30–60 cm), although the abundance of Trichoderma was not significantly different between the oil palm and non-oil palm ecosystems. Characterisation of Trichoderma isolates based on cultural, morphological and DNA polymorphism showed that T. harzianum, T. virens, T. koningii and T. longibrachiatum made up 72, 14, 10 and 4% of the total Trichoderma isolates isolated. As Trichoderma species are present in the oil palm ecosystem, but at lower numbers and in locations different from those desired, soil augmentation with antagonistic Trichoderma spp. can be developed as a strategy towards integrated management of basal stem rot of oil palm.     

Simultaneous CRISPR/Cas9‐mediated editing of cassava eIF4E isoforms nCBP‐1 and nCBP‐2 reduces cassava brown streak disease symptom severity and incidence

Cassava brown streak disease (CBSD) is a major constraint on cassava yields in East and Central Africa and threatens production in West Africa. CBSD is caused by two species of positive‐sense RNA viruses belonging to the family Potyviridae, genus Ipomovirus: Cassava brown streak virus (CBSV) and Ugandan cassava brown streak virus (UCBSV). Diseases caused by the family Potyviridae require the interaction of viral genome‐linked protein (VPg) and host eukaryotic translation initiation factor 4E (eIF4E) isoforms. Cassava encodes five eIF4E proteins: eIF4E, eIF(iso)4E‐1, eIF(iso)4E‐2, novel cap‐binding protein‐1 (nCBP‐1), and nCBP‐2. Protein–protein interaction experiments consistently found that VPg proteins associate with cassava nCBPs. CRISPR/Cas9‐mediated genome editing was employed to generate ncbp‐1, ncbp‐2, and ncbp‐1/ncbp‐2 mutants in cassava cultivar 60444. Challenge with CBSV showed that ncbp‐1/ncbp‐2 mutants displayed delayed and attenuated CBSD aerial symptoms, as well as reduced severity and incidence of storage root necrosis. Suppressed disease symptoms were correlated with reduced virus titre in storage roots relative to wild‐type controls. Our results demonstrate the ability to modify multiple genes simultaneously in cassava to achieve tolerance to CBSD. Future studies will investigate the contribution of remaining eIF4E isoforms on CBSD and translate this knowledge into an optimized strategy for protecting cassava from disease.     

Antibodies to Pseudogymnoascus destructans are not sufficient for protection against white‐nose syndrome

White‐nose syndrome (WNS) is a fungal disease caused by Pseudogymnoascus destructans (Pd) that affects bats during hibernation. Although millions of bats have died from WNS in North America, mass mortality has not been observed among European bats infected by the fungus, leading to the suggestion that bats in Europe are immune. We tested the hypothesis that an antibody‐mediated immune response can provide protection against WNS by quantifying antibodies reactive to Pd in blood samples from seven species of free‐ranging bats in North America and two free‐ranging species in Europe. We also quantified antibodies in blood samples from little brown myotis (Myotis lucifugus) that were part of a captive colony that we injected with live Pd spores mixed with adjuvant, as well as individuals surviving a captive Pd infection trial. Seroprevalence of antibodies against Pd, as well as antibody titers, was greater among little brown myotis than among four other species of cave‐hibernating bats in North America, including species with markedly lower WNS mortality rates. Among little brown myotis, the greatest titers occurred in populations occupying regions with longer histories of WNS, where bats lacked secondary symptoms of WNS. We detected antibodies cross‐reactive with Pd among little brown myotis naïve to the fungus. We observed high titers among captive little brown myotis injected with Pd. We did not detect antibodies against Pd in Pd‐infected European bats during winter, and titers during the active season were lower than among little brown myotis. These results show that antibody‐mediated immunity cannot explain survival of European bats infected with Pd and that little brown myotis respond differently to Pd than species with higher WNS survival rates. Although it appears that some species of bats in North America may be developing resistance to WNS, an antibody‐mediated immune response does not provide an explanation for these remnant populations.     

Water balance in the arborescent palm, Sabal palmetto. II. Transpiration and stem water storage

The contribution of stem water storage to the water balance of the arborescent palm, Sabal palmetto, was investigated using greenhouse studies, field measurements and a tree-cutting experiment. Water balance studies of greenhouse trees (1.5 to 3 m tall) were conducted in which transpiration was measured by weight loss, and changes in soil and stem water content by time-domain reflectometry. When the greenhouse plants were well-watered (soil moisture near saturation), water was withdrawn from the stem during periods of high transpiration and then replenished during the night so that the net transpirational water loss came primarily from the soil. As water was withheld, however, an increasing percentage of daily net transpirational water loss came from water stored in the stem. However, studies on palms growing in their natural environment indicated that during periods of high transpiration leaf water status was somewhat uncoupled from stem water stores. In a tree-cutting experiment, the maintenance of high relative water content of attached leaves was significantly correlated with stem volume/leaf area. Leaves of a 3-m tree remained green and fully hydrated for approximately 100d after it had been cut down, whereas those of a 1-m-tall plant turned brown within one week. The significance of stem water storage may be in buffering stem xylem potentials during periods of high transpiration and in contributing to leaf survival during extended period of low soil water availability.