Foliar sprays of NPK fertilizers induce systemic protection againstPuccinia sorghi andExserohilum turcicum and growth response in maize

One spray of 0.1 M aqueous solutions of NPK fertilizers on the upper sides of maize leaves 1, 2, and 3, 2–4 h prior to inoculation, induced systemic resistance (ISR) against northern leaf blight (NLB) caused byExserohilum turcicum andPuccinia sorghi which were developed on leaves 4, 5, 6, and 7. ISR was expressed as a reduction in the number and area of lesions ofE. turcicum and in the number of sporulating or non-sporulating pustules ofP. sorghi on leaves 4, 5, 6, and 7. The reduction in the number of NLB lesions ranged from 51% (KH₂PO₄) to 69% (K₂HPO₄) and their size reduction ranged from 73% (KNO₃) to 91% (K₂HPO₄) as compared with water prayed plants. The reduction in the number of pustules ofP. sorghi ranged from 66 to 77%. Fertilizers consisting of various combinations of N, P and K in every case induce similar levels of protection in either host-pathogen system. The induced protection was evident regardless of the leaf position or the rate of NPK accumulation in the upper protected leaves. High fresh weight was detected in the induced plants which expressed the greatest induced protection against NLB and common rast. The possible dual use of NPK fertilizers — to supply nutrients to plants foliarly and at the same time to activate the mechanism(s) for induction of systemic protection toP. sorghi andE. turcicum in maize — is discussed.Contribution from the Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel, No. 1315-E, 1995 series.     

Epidemiology of Northern leaf blight on sweet corn

The epidemiology of northern leaf blight of corn, caused byExserohilum turcicum (Pass.) Leonard and Suggs, is reviewed. The minimal dew period required for infection is temperature-dependent. At 25°C, 1 h of dew is sufficient to cause infection and at this temperature the minimal dew period for sporulation is 14 h. Under natural conditions when one dew night is not long enough for conidia to develop, the dew period on the following night enables the completion of conidial formation. The amount of conidia formed is dependent on temperature, light, plant age, leaf position and plant susceptibility. Both qualitative and quantitative types of resistance were identified in several hybrids. Subsequently, there developed additional biotypes ofE. turcicum which are aggressive to plants containing qualitative monogenic resistance. Within the same physiological race, a significant variation in aggressiveness between isolates from various locations is observed. The pathogen overwinters as mycelia and conidia in infected leaves, husks and other plant parts, or onSorghum halepense L. Reduction in yield due to northern leaf blight is associated with the level of resistance of the host plant, with disease severity, plant age during infection, and position of infected leaves.     

The overwintering ofExserohilum turcicum in Israel

Exserohilum turcicum (Pass.) Leonard and Suggs, the causal agent of northern leaf blight of corn, overwinters onSorghum halepense L. plants and on corn debris (dead leaves). Spqrulating lesions ofE. turcicum were observed on sorghum plants in the winter (February). Spores from these lesions were pathogenic to susceptible sweet corn plants cv. ‘Jubilee’. Infected sporulating leaves of corn were stored for 1 year at 20°C (40-60% relative humidity), at 5°C (60% relative humidity), or buried 5 cm underground. During the storage period, 32% and 22% of the spores formed chlamydospores, at 20° and 5°C, respectively. Leaves buried 5 cm underground were totally decomposed after 6 months. After 4 months, 25% of the spores in the buried leaves had formed chlamydospores. Spores with chlamydospores were pathogenic to corn plants. The viability of spores without chlamydospores stored at 20°, 5°C or buried underground was 0, 60 and 0%, respectively. In a parallel experiment infected leaves were stored under similar conditions and allowed to sporulate. No sporulation occurred on infected leaves buried in soil. Spores produced on infected leaves stored at 20° and 5°C were highly pathogenic to corn plants. In leaves treated with 0.1N glucose, chlamy dospore formation was significantly inhibited.     

Involvement of a phytotoxic peptide in the development of the Northern leaf blight of corn

Non pathogenic isolates ofExserohilum turcicum successfully infect corn plants in the presence of syntheticE. turcicum toxin during inoculation. The toxin significantly increased the number of appressoria and the ramification of germinating conidia both on host leaves and on artificial media. These findings indicate that this toxin plays an important role in infection of Northern leaf blight.This article is dedicated to the memory of Yehouda Levy who passed away recently.     

Physiological races ofExserohilum turcicum in Israel

A set of differentials of corn plants(Zea mays L.) containing Ht1, Ht2, Ht3 or HtN genes was used to identify races ofExserohilum turcicum in Israel. Plants were inoculated with 14 isolates ofE. turcicum collected from various regions in Israel (from Ayyelet HaShahar in the north to Sa’ad in the south). Differentials containing Ht1, Ht2, Ht3 or HtN genes were resistant to the 14 isolates tested, whereas the inbred lines without Ht genes were highly sensitive. Resistance was characterized by the formation of non-sporulating chlorotic lesions. When plants containing Ht1, Ht2 or Ht3 genes were inoculated with relatively high inoculum concentrations (over 50 conidia/drop), chlorotic lesions were associated with necrosis in the center of the lesions. Sporulation of the fungus in the necrotic parts of the lesions was significantly less than on plants without Ht genes. No necrosis was observed in plants with the HtN gene. Our results indicate that the physiological race ofE. turcicum in Israel is race 1.     

Root rot of hyacinths caused by species of Pythium

The root rot widely seen in hyacinth was found to be caused byPythium spp. instead of byFusarium culmorum. Of sixPythium species isolated, three, includedP. ultimum andP. violae, were investigated in glasshouse experiments and their pathogenicity demonstrated. In these experiments plants were successfully grown in containers in which an aqueous mist was maintained, or in water cultures. In experimental plots on infected soil, Dexon, a fungicide selective for Pythiaceae, distinctly reduced the number of dead plants and increased yield and bulb size, thus confirming the role ofPythium in causing root rot. Practical application of Dexon deserves further attention.     

Systemic infection of sorghum and corn by conidia ofSclerospora Sorghi

Conidia ofSclerospora sorghi, obtained from either systemically-infected or local-lesion-infected leaves of sorghum (cv. Vidan), were capable of inducing typical downy mildew systemic infection, including oospore formation, in sorghum and corn hybrids. Very young inoculated seedlings displayed chlorotic systemic symptoms already on the first leaf, and often died at fourth-leaf stage. Systemic infection was induced by conidia on sorghum 1–14 days old at inoculation. Incidence of infection was much higher and symptoms less delayed when the shoot rather than coleorhizas of young sorghum and corn seedlings were inoculated; in two-week-old sorghum with three leaves, inoculation of the coleoptile or of the base of the second and third blades resulted in systemic infection; with coleoptile inoculation partial leaf chlorosis was delayed until the fourth-or fifth-leaf stage, showing that penetration without symptoms had occurred as far as the meristematic tissues of young leaves still within the leaf tube. Conidial inoculation of young sorghum tillers sprouting after cutting down healthy mother shoots resulted in systemic infection. Conidial inoculum is deemed to be the probable major means for systemic infection of corn and sorghum sown in fields in which oospores are not present; inoculation of new tillers of forage sorghum by conidia from infected plants in a neighboring field can explain the rise in numbers of plants systemically stricken. Two sweet corn hybrids — one considered resistant in the field, the other very susceptible — proved equally susceptible when inoculated with conidia at 5 days of age.     

Stage specificity of catalase isoform activity in Exserohilum turcicum

Exserohilum turcicum is the fungal agent that causes northern leaf blight disease in maize. Spores of E. turcicum can germinate in water containing up to 15 m M hydrogen peroxide. Initially the catalase isoform activities from E. turcicum cultured on artificial medium were analysed by polyacrylamide gel electrophoresis. Seven different catalase isoform activities were detected during a 72-hour time course from spore germination to young hyphal formation. During the time period studied the activity levels of each isoform varied independently. In two-week-oldE. turcicum mycelia, only the activities of catalase isoforms #2 and #3 were detected. During a compatible interaction on maize leaves, plant catalases were suppressed with E. turcicum isoform #3 being detected from 72 h after inoculation onwards. E. turcicum #2 was the predominant isoform detected in necrotic lesions. Salicylic acid was not found to effect fungal or maize catalase activities. E. turcicum isoform #3 activity was found to be strongly induced by hydrogen peroxide and by the herbicide 3-amino-1,2,4-triazole (AT), while the activity of all other isoforms was suppressed by AT.     

Maize resistance to northern corn leaf blight is potentiated by nickel

Northern corn leaf blight (NCLB), caused by Exserohilum turcicum, is one of the most devastating diseases affecting maize yield worldwide. A foliar spray of nickel (Ni) to potentiate maize resistance against NCLB was investigated by examining alterations in the photosynthetic apparatus (leaf gas exchange and chlorophyll [Chl] a fluorescence parameters), production of ethylene and reactive oxygen species as well as activities of defence and antioxidant enzymes. Mycelial growth of E. turcicum was inhibited by Ni in vitro. Inoculated plants sprayed with Ni exhibited higher foliar Ni concentration, reduced NCLB symptoms, and lower concentrations of malondialdehyde and hydrogen peroxide. In inoculated leaves of plants not sprayed with Ni, concentrations of Chl a, Chl b, and carotenoids were lower and the photosynthetic apparatus was impaired at the biochemical level due to high NCLB severity. The activities of antioxidant enzymes were not affected by Ni, except an increase in glutathione reductase activity for noninoculated plants sprayed with Ni. High lipoxygenase and polyphenoloxidase activities, lower ethylene production, as well as elevated production of phenolics and lignin helped decrease NCLB severity in the leaves of Ni-sprayed plants.     

Corynebacterium fascians andBotrytis cinerea inPelargonium zonale an aspect from the many factors causing the wilting of pelargonium

The investigations on the cause of the “Lent disease” of mother plants as well as cuttings ofPelargonium zonale have indicated, that these troubles are due to faulty cultural methodes combined with an infection byCorynebacterium fascians and “weakness parasites”. The bacteria cause the axillary buds to develop leafy galls on the stem under the soil level and small tumorous growths on the cortex of the stem above, the socalled corky spots. The infected plants may not wither, but galls and spots are more easily infected by pathogens than healthy plants. FrequentlyBotrytis cinerea infects these tissues and causes the black base rot. Spraying the mother plants with streptomycin (Fytostrep 60: 2.5 ml/l) and thiram (0.2%) decreased the base rot, but markedly inhibited the rooting of the cuttings. Moreover, the inocula of bothC. fascians andB. cinerea are present in such a large quantity, that full control is impossible. Reduction of losses may be achieved by improvement of cultural methods and soil disinfection.     

Effects of organic wastes,Glomus intraradices andPseudomonas putida on the growth of tomato and on the reproduction of the Root-knot nematodeMeloidogyne incognita

Effects of organic wastes (biosolids, horse manure, sawdust and neem leaf litter [NLL]), an arbuscular mycorrhizal (AM) fungusGlomus intraradices, and a plant growth-promoting rhizobacteriumPseudomonas putida, were studied on the growth of tomato and on the reproduction ofMeloidogvne incognita. Pseudomonas putida andG. intraradices promoted tomato growth in nematode-infected and nematode-free plants but growth promotion was higher in the infected ones. WhenP. putida andG. intraradices were applied together, the increase in tomato growth was greater than when either agent was applied alone. Of the organic wastes, NLL was better in improving tomato growth of nematode-infected plants followed by biosolids, horse manure and sawdust. Combined use of NLL withP. putida plusG. intraradices was best in improving growth of the infected plants. Root colonization byP. putida was increased more when inoculated withG. intraradices than when inoculated singly. Of the organic wastes, use of sawdust withP. putida caused a greater increase in root colonization by fluorescent pseudomonads followed by NLL, horse manure and biosolids. Nematode parasitism had an adverse effect on root colonization byP. putida. Inoculation ofP. putida and organic wastes increased the root colonization caused by the AM fungus.P. putida was better in reducing galling and nematode multiplication thanG. intraradices, whereas use of the two together was better than that of either of them alone. Among organic wastes, NLL was better in reducing galling and nematode multiplication followed by biosolids, horse manure and sawdust. Combined use of NLL withP. putida plusG. intraradices was better in reducing galling and nematode multiplication than any other treatment.     

Species of Pythium in the Netherlands

A review is given on observations ofPythium spp. so far published. In personal investigations about 4000 isolates ofPythium species were obtained from soil, water, Daphnias and plants during 1966–1972. Special attention was paid to the progressive colonization of the newly reclaimed polder Zuidelijk Flevoland: the numbers of species per sample increased from 0 in 1966 to 13 in 1972. The percentage of samples which did not contain anyPythium decreased from 54 in 1968 to 0 in 1972. From Zuidelijk Flevoland altogether 21 species were isolated, from Oostelijk Flevoland 22, of which 19 occurred in both the polders. The soil depth down to 10 cm had little influence on the occurrence ofPythium species. From soils near Rhenen and Zutphen 12 and 19 species respectively were isolated, from flax and flaxfields 11, from forests 8, from roots of different plants 17 and from water 11. Besides species already previously recorded, 15 new records ofPythium species for the Netherlands are documented. Species with filamentous non-swollen sporangia preferentially occurred in water and wet soils while species with spherical sporangia or hyphal swellings dominated in cultivated soils, forests and in the roots of phanerogams. In 1968 species with filamentous non-swollen sporangia comprised 89% of the isolations from Zuidelijk Flevoland; this percentage decreased to 9 in 1972. The percentage of isolates with spherical sporangia or hyphal swellings from cultivated soils, forest soils or plant material was mostly over 90.P. rostratum Butler, though found in all kinds of soils, was often the only species which could be isolated from dry sandy soil samples.P. sylvaticum Campbell & Hendrix was the most common species in cultivated soils in the Netherlands (37% of the isolates in 1968–1972), followed byP. oligandrum Drechsler (22%) andP. paroecandrum Drechsler (11.5%).     

Biological control ofBotrytis cinerea in residues and flowers of Rose (Rosa hybrida)

Microbial isolates from living petals, petal residues and leaf residues of rose, and from laboratory collections, were evaluated for control ofBotrytis cinerea in rose. In leaf residues artificially infested withB. cinerea, isolates of the filamentous fungiGliocladium roseum, FR136 (unidentified) andTrichoderma inhamatum reduced sporulation of the pathogen by >90%, other filamentous fungi were 25–90% effective, and those of yeasts and bacteria were <50% effective. In artificially inoculated petal residues, no microbe reduced sporulation ofB. cinerea by >75%, but isolates ofCladosporium oxysporum and four yeasts were 51–75% effective, and three filamentous fungi, eight yeasts andBacillus subtilis isolates were 26–50% effective. Isolates ofT. inhamatum, C. oxysporum andG. roseum performed best againstB. cinerea among isolates evaluated in leaf residues naturally infested with the pathogen and indigenous microorganisms. Totals of ten isolates of filamentous fungi (includingC. oxysporum andC. cladosporioides), two of yeasts and five ofBacillus subtilis completely prevented lesion production byB. cinerea in detached petals, and a further six isolates of filamentous fungi (includingG. roseum) and six yeasts were 90–99% effective. Isolates ofC. oxysporum, C. cladosporioides andB. subtilis, the most effective microorganisms againstB. cinerea in flower buds, reduced number of lesions in the range of 42–65% compared with 59–89% for à standard fungicide (vinclozolin). It is suggested that application of leading antagonists Jo living rose leaves and flowers should optimize control of inoculum production byB. cinerea when the tissues die. Optimal biocontrol of lesion production in flower buds requires a better understanding of the microenvironment of petals.     

Tentative description of Hippeastrum latent virus in Hippeastrum hybridum plants and differentiation from Hippeastrum mosaic virus

In mosaic-diseased plants ofHippeastrum hybridum two viruses were found. One virus with a normal length of 706 nm caused local lesions onHyoscyamus niger test plants and mosaic symptoms in the leaves ofH. hybridum. This virus was identified with theHippeastrum mosaic virus (HMV) (*/*∶*/*∶E/E∶S/*) and had a dilution end point between 10^?3 and 10^?4, a thermal inactivation point between 55–60°C and a longevity at room temperature of 28–32 hours. The second virus had a normal length between 584 and 611 nm depending on the method used. It caused local lesions onGomphrena globosa andChenopodium quinoa leaves, and after inoculation ofH. hybridum was found to be present without showing symptoms. It was readily purified from inoculated leaf tissue ofC. quinoa andNicotiana clevelandii by differential centrifugation and ofH. hybridum by density-gradient centrifugation. Purified virus had an absorption minimum at 242 nm, a maximum at 262 nm and a 260/280 absorption ratio of 1.19. The dilution end point was between 10^?3 and 10^?4, the thermal inactivation point between 70 and 80°C and the longevity in vitro at room temperature 28–32 hours. Although no direct comparisons have been made with other members of the potexvirus group, the virus seems to be a new one now namedHippeastrum latent virus. Both viruses were not seed-borne.     

Surveys of cereal diseases and pests in the Netherlands. 3. Monographella nivalis and Fusarium spp. in winter wheat fields and seed lots

Data from surveys of winter wheat fields in the period 1974–1986 and of seed lots in the period 1962–1986 and identifications of diseases on plant samples were compiled to describe the occurrence of snow mould (Monographella nivalis) andFusarium spp. On average,M. nivalis dominated overFusarium spp. The complex ofFusarium spp. constituted mainly ofF. culmorum, followed byF. avenaceum andF. graminearum. M. nivalis was dominant in May on stem-bases and in July on leaves and leaf sheaths. On seedsM. nivalis predominated only in years with low temperatures in July and August.Average brown footrot infection in the field was 4% tillers in May and 5% culms in July. Brown footrot intensity in July was high in cropping seasons with high precipitation in October and with low temperatures in October, November and December. In July during the early eighties, an average of 8% of leaves and 6% of flag leaf sheaths were infected byM. nivalis. Average ear blight incidence was 1.2% glumes infected. Seed contamination by these pathogens averaged 16% in the years 1962–1986. The contamination was high in years with high precipitation in June, July and August. Aspects of cv. resistance and yield loss are illustrated.     

Induced resistance againstPhytophthora capsici in pepper plants in response to DL-β-amino-n-butyric acid

Treatment of pepper plants with the nonprotein amino acid, DL-ß-amino-n-butyric acid (BABA) induced resistance to subsequent infection byPhytophthora capsici. In contrast, theα-, andγ-isomers of aminobutyric acid were ineffective as inducers of resistance. A relatively high concentration of BABA at 1,000μg ml^?1, which had no antifungal activityin vitro againstP. capsici, was required to induce resistance against Phytophthora blight with a foliar and stem spray, thus leading to complete control of the disease. About 1 day interval between BABA-treatment and challenge inoculation was sufficient to induce resistance in pepper plants. High inoculum levels ofP. capsici caused Phytophthora development slowly in pepper stems treated with BABA, especially at early plant growth stage, which suggests that the induced resistance in pepper plants may be more quantitative rather than qualitative. BABA applied to the root system also protected pepper stems fromP. capsici infection.     

Signal interactions in pathogen and insect attack: systemic plant-mediated interactions between pathogens and herbivores of the tomato,Lycopersicon esculentum

Plant-mediated interactions (i.e., induced resistance) between plant pathogens and insect herbivores were investigated using several pests of the cultivated tomato,Lycopersicon esculentum. Single leaflets of tomato leaves were injured by allowing a third-instarHelicoverpa zealarva to feed on the leaflets or by inoculating the leaflets withPseudomonas syringaepv.tomato(the causal agent of bacterial speck in tomato;Pst) or withPhytophthora infestans(the causal agent of late blight). Leaflets on separate plants were sprayed with benzothiadiazole, a chemical inducer of resistance toPst. The effects of these treatments on the resistance of uninoculated or undamaged leaflets to bothPstandH. zeawere then assessed after appropriate periods of time. The levels or activities of several defense-related proteins were determined in parallel. Infection of leaflets byPstdecreased the suitability of uninoculated leaflets of the same leaf for bothH. zeaand forPst. Similarly, feeding byH. zeacaused leaf-systemic increases in resistance to bothH. zeaandPst. Infection of leaflets byP. infestans, in contrast, had no effect on resistance of leaflets toH. zea. Treatment of leaves with benzothiadiazole induced resistance toPstbut improved suitability of leaflets forH. zea. Feeding byH. zeacaused the systemic accumulation of proteinase inhibitor mRNA and the systemic induction of polyphenol oxidase activity; in contrast, treatment with benzothiadiazole and inoculation withP. infestanscaused the systemic accumulation of pathogenesis-related protein mRNA and the systemic induction of peroxidase activity. Inoculation of leaflets withPstcaused the leaf-systemic accumulation of both pathogenesis-related protein and proteinase inhibitor mRNA and the systemic induction of both peroxidase and polyphenol oxidase activity. These results provide clear evidence for reciprocal induced resistance involving certain pathogens and arthropod herbivores of tomato. In addition, these results provide several insights into the integration and coordination of the induced defenses of tomato against multiple pests and suggest that the expression of resistance against some pests may compromise resistance to others.     

Negatively correlated cross-resistance to dodine in fenarimol-resistant isolates of various fungi

The majority of fenarimol-resistant laboratory isolates ofAspergillus nidulans, Cladosporium cucumerinum, Penicillium expansum, P. italicum, andUstilago maydis testedin vitro, displayed a moderate degree of negatively correlated cross-resistance to dodine. A limited number of isolates also possessed an increased sensitivity to guazatine. Colonies of fenarimol-resistant isolates ofA. nidulans with increased sensitivity to dodine showed sector formation on dodine-amended agar. Subcultures from these sectors appeared to have the wild-type sensitivity to dodine and fenarimol. The results indicate that fenarimol resistance and increased sensitivity to dodine are closely linked. The potential practical significance of the results is discussed.     

Biological control ofBotrytis cinerea on tomato stem wounds withTrichoderma harzianum

The effectiveness ofTrichoderma harzianum in suppression of tomato stem rot caused byBotrytis cinerea was examined on tomato stem pieces and on whole plants. Ten days after simultanous inoculation withB. cinerea andT. harzianum, the incidence of infected stem pieces was reduced by 62–84%, the severity of infection by 68–71% and the intensity of sporulation by 87%. Seventeen days after inoculation of wounds on whole plants, the incidence of stem rot was reduced by 50 and 33% at 15 and 26 °C, respectively, and the incidence of rot at leaf scar sites on the main stem was reduced by 60 and 50%, respectively. Simultanous inoculation and pre-inoculation withT. harzianum gave good control ofB. cinerea (50 and 90% disease reduction, 10 days after inoculation). The rate of rotting was not reduced by the biocontrol agent once infection was established. However, sporulation byB. cinerea was specifically reduced on these rotting stem pieces. Temperature had a greater effect than vapour pressure deficit (VPD) on the efficacy of biocontrol. Suppression ofB. cinerea incidence byT. harzianum on stem pieces was significant at 10 °C and higher temperatures up to 26 °C. Control of infection was significantly lower at a VPD of 1.3 kPa (60% reduction), than at VPD<1.06 kPa (90–100% control). Reductions in the severity of stem rotting and the sporulation intensity of grey mould were generally not affected by VPD in the range 0.59–1.06 kPa. Survival ofT. harzianum on stems was affected by both temperature and VPD and was greatest at 10 °C at a low VPD and at 26 ° C at a high VPD.