M. Reuveni

Foliar-fertilizer therapy — a concept in integrated pest management

Abstract Heavy agricultural use of fertilizers has been implicated in degradation of the environmental quality of lakes, rivers and aquifers. There is also widespread public concern about the use of pesticides, including fungicides, on farms and their potential effect on our environment and food. However, it is certain that the use of fungicides as part of intensive agriculture has stabilized our food supply and permitted millions of people to live longer lives. Data from our laboratory and others have indicated that foliar sprays of phosphate and potassium salts can induce systemic protection against foliar pathogens in various crops such as cucumber, maize, rose, grapevine, apple, mango and nectarine. Expression of disease tolerance is dependent on a number of factors including use of fertilizers and pesticides. Therefore, the possible dual role of NPK fertilizers in activation of the mechanism(s) which induce plant protection against pathogens was studied. Data from the application of this concept to various host-pathogen interactions are presented in the present review. A single phosphate spray of 0.1 M solution induced a systemic protection against powdery mildew in cucumber caused by Sphaerotheca fuliginea and against common rust in maize caused by Puccinia sorghi , and northern leaf blight (NLB) caused by Exserohilum turcicum . This systemic protection against powdery mildew in cucumber, common rust or NLB in maize was obtained on upper leaves after NPK fertilizer application on the lower leaves. In both the latter host-pathogens interactions, growth increase was also observed in maize plants as a result of one foliar spray of phosphates. In addition, it was evident throughout all the experiments that a single application of phosphates was effective in suppressing the lesions of powdery mildew on the diseased foliage of cucumber, greenhousegrown roses, field grown mango, nectarine and grapevine. This phenomenon was investigated in combination with fungicides.

Local and Systemic Activity of BABA (DL-3-aminobutyric Acid) Against Plasmopara viticola in Grapevines

The non-protein amino acid BABA (DL-3-amino-n-butanoic acid, β-aminobutyric acid) is reported here to induce local and systemic resistance against downy mildew in grape leaves. Leaf discs of susceptible cultivars placed on BABA solutions and inoculated with Plasmopara viticola on the counter surface produced brownish restricted lesions below the inoculation site (Hypersensitive-like response, HR) which failed to support fungal sporulation. Histochemical analyses of such HR lesions revealed the accumulation of lignin-like deposits in the host cells. In contrast, water-treated inoculated discs produced expanded chlorotic lesions with profuse sporulation in which no lignin accumulation was observed. Mock-inoculated BABA-treated leaf discs showed no HR or lignin accumulation. Concentrations as low as 25 µg/ml (0.25 mM) of BABA sufficed to prevent tissue colonization with the fungus. Five other isomers of aminobutyric acid, namely L-2 aminobutyric acid, 2-amino isobutyric acid, DL-2-aminobutyric acid (AABA), DL-3-amino isobutyric acid, and 4-aminobutyric acid (GABA) gave no protection against the downy mildew fungus. Of the two (R and S) enantiomers of BABA only the R form was active in producing HR, suggesting a specific stereostructure requirement for activity. BABA could stop fungal colonization even when applied post-infectionally to leaf discs. Resistance of BABA-pulse-loaded leaf discs persisted for more than 14 days. BABA provided systemic protection against the disease when applied via the root system or via the lower leaves of grape plants. Application of 14C-BABA to a single leaf of intact plants showed the accumulation of the 14C label in upper leaves (and root tips), suggesting sink-oriented transport.

Controlling powdery mildew caused by Sphaerotheca fuliginea in cucumber by foliar sprays of phosphate and potassium salts

Abstract Powdery mildew in greenhouse-grown cucumber (Cucumis sativus L.), caused by Sphaerotheca fuliginea (Schlecht.:Fr.) Pollacci, was significantly controlled by a foliar spray pre-inoculation treatment of 20 mM aqueous solutions of either monopotassium phosphate or potassium nitrate, or a 0.01% solution of the systemic fungicide pyrifenox. Further foliar sprays of mono- or dipotassium phosphate, sodium bicarbonate (1%) and pyrifenox remarkably suppressed powdery mildew colonies on diseased foliage. Solutions of 25 mM of mono- or dipotassium phosphates and potassium nitrate applied on a 7 or 14 day schedule were highly protective against natural infection by the mildew fungus in the greenhouse. Pyrifenox was not significantly more effective in controlling S. fuliginea than either phosphate or the potassium salts. These salt solutions were not phytotoxic to plant foliage. The inhibitory effectiveness of phosphate and potassium salts makes them useful biocompatible fungicides and possibly ideal foliar fertilizers for disease control in the greenhouse.

Efficacy of trifloxystrobin (Flint), a new strobilurin fungicide, in controlling powdery mildews on apple, mango and nectarine, and rust on prune trees.

Trifloxystrobin is a new strobilurin fungicide, active against a wide range of fungal plant pathogens. The present study shows that trifloxystrobin is highly effective in controlling powdery mildews on field-grown apple, mango and nectarine trees and rust disease on prune trees. In the majority of the field experiments conducted during 1998–1999, trifloxystrobin (at a concentration of 0.01–0.015% v/v) was superior to demethylation inhibitors (DMI fungicides) or sulphur. Trifloxystrobin was significantly (p<0.05) more effective than the DMI fungicide penconazole in controlling primary and secondary infections of powdery mildew on apple trees, and it provided 95–100% protection compared with non-treated trees. Similarly, trifloxystrobin was more effective than either myclobutanil or penconazole in controlling powdery mildew on fruit and leaves of nectarine trees. Trifloxystrobin provided the best control of powdery mildew on bloom clusters of mango, while sulphur, hexaconazole, and kresoxim-methyl were less effective. Trifloxystrobin effectively inhibited rust on prune trees and provided 98.0% protection, compared with the control. Trifloxystrobin is an excellent compound for efficient integration into control programs against fungal pathogens in apple, mango and stone fruit orchards.

Efficacy of foliar sprays of phosphates in controlling powdery mildews in field-grown nectarine, mango trees and grapevines

Abstract Foliar sprays of 0.025M and 0.04M solutions of K 2 HPO 4 and KH 2 PO 4 + KOH (both plus Triton X-100) and commercial systemic fungicides inhibited development of powdery mildew fungi on fruit clusters, flower clusters, fruits and leaves of field-grown grapevines, mango and nectarine. The effectiveness of phosphates in controlling powdery mildew on berries of Chardonnay grapevines was similar to that of the systemic fungicide pyrifenox (Dorado 480 EC). However, the systemic fungicides diniconazole (Marit 12.5% WP), myclobutanil (Sisthane 12E) and penconazole (Ophir), were more effective in controlling the disease on inflorescences of mango and fruits of nectarine, respectively, than either phosphate. Alternating treatments of phosphate salt with each of these fungicides, however, enhanced the inhibitory effect against the fungus in each crop. With the exception of young, newly developed nectarine leaves, these phosphate solutions were not phytotoxic to plant tissue. The inhibitory effectiveness of phosphate salts makes them useful ‘biocompatible’ fungicides and ideal foliar fertilizers for field application for disease control.

Controlling downy mildew (Plasmopara viticola) in field-grown grapevine with β-aminobutyric acid (BABA)

Two foliar sprays of BABA (DL-3-amino-n-butanoic acid, DL-β-aminobutyric acid), or a mixture of BABA and different fungicides at reduced rates, effectively controlled (>90%) downy mildew, caused byPlasmopara viticola, in the foliage of field-grown grapevines. In five field trials BABA sprays resulted in a significant reduction of infectious leaf area and fungal sporulation, and of necrosis of oilspots. The level of disease control for BABA in four trials was similar to that achieved by metalaxyl-Cu or ‘Acrobat Plus’ (dimethomorph + mancozeb). Two-way tank-mixtures of BABA + fosetyl-AI, BABA + folpet, or BABA + Bion (benzothiadiazole), each at half the recommended rate, provided an additive effect againstP. viticola, performing as well as the full rate of each fungicide alone. BABA was not phytotoxic and did not affect pH, total titratable acids, or °Brix of the juice, as determined by commercial fungicidal standards. The results indicate that foliar applications of BABA can efficiently be integrated into a downy mildew control program in vineyards.

Activity of the New Fungicide Benthiavalicarb Against Plasmopara viticola and its Efficacy in Controlling Downy Mildew in Grapevines

Benthiavalicarb is a new fungicide active against Oomycetes fungal plant pathogens. The present study shows that benthiavalicarb is effective for controlling the Oomycete fungal pathogen Plasmopara viticola, which causes downy mildew in grapevines. The fungicide did not affect zoospore discharge from sporangia of P. viticola, but strongly inhibited zoospore encystment, cystospore germination in vitro and mycelial growth, together with sporangial production in vivo. Benthiavalicarb showed strong prophylactic and local activity in intact plants or detached leaves and low translaminar activity. The compound was not translocated from leaf to leaf in either a acropetal or basipetal direction. Benthiavalicarb applied at 1, 3 and 6 days post-inoculation protected grapevine plants against downy mildew and inhibited sporulation of the pathogen. Similar results were obtained on leaf disks if benthiavalicarb was applied up to 96 h post-inoculation. Benthiavalicarb diminished the sporulation of P. viticola when applied to established disease in the tissue. Benthiavalicarb remained active on leaves for a period up to 28 days. Two foliar applications of benthiavalicarb, 2 weeks apart, to field-grown grapevines inhibited downy mildew development and were as effective as the standard metalaxyl-Cu treatment in controlling the disease. A formulated mixture of benthiavalicarb + Folpet was similar or superior in performance to metalaxyl-Cu and the new strobilurin trifloxystrobin in controlling downy mildew. The effectiveness of benthiavalicarb makes it well suited for integration into a control programme against downy mildew disease in vineyards, and as a component to delay resistance buildup.

Local and systemic control of powdery mildew (Leveillula taurica) on pepper plants by foliar spray of mono-potassium phosphate

Abstract A foliar spray of 1% (w/v) solution of the fertilizer mono-potassium phosphate (MKP) (KH 2 PO 4 ) on the upper surfaces of lower leaves of greenhouse-grown peppers induced local and systemic control against Leveillula taurica , as compared with control plants. This protection was expressed by a reduction in the leaf area covered with sporulating colonies and in conidial production on leaf tissue, 24 or 48 h post-treatment, when MKP was applied on lower leaves of plants that had been exposed to the source of inoculum. Foliar application of MKP, initiated before or after exposure to heavily diseased plants as the source of inoculum, was effective in controlling powdery mildew. Application of MKP efficiently suppressed powdery mildew as expressed by inhibition of the development of new sporulating colonies, as well as the conidial production of the fungus on infected tissue. Microscopic examination indicated destruction of both hyphae and conidial structures on MKP-treated leaves. The efficacy of MKP in controlling powdery mildew on greenhouse-grown plants was compared with a sterol-inhibiting systemic fungicide. Both treatments significantly inhibited powdery mildew as compared with non-treated control plants, although the fungicide-based treatment seemed to be slightly more effective (not significant) in controlling the disease. Phosphate solutions were not phytotoxic to plant tissue and did not affect the yield, as compared with the fungicide treatment. However, a lower yield was recorded for the non-treated control plots because of mildew infection on leaves. These data indicate that MKP spray may be applied as an alternative practice for the control of powdery mildew in peppers.

A foliar spray of micronutrient solutions induces local and systemic protection against powdery mildew (Sphaerotheca fuliginia) in cucumber plants

A single spray of solutions of 0.005M H3BO3, 0.0025M CuSO4, and 0.0025 MnCl2, on the upper surface of the first true leaf of cucumber plants 2 h before inoculation with a conidial suspension of Sphaerotheca fuliginea, induced systemic protection against powdery mildew in leaves 2 and 3 without causing any damage on the induced leaf (first leaf). A similar level of systemic protection was observed when plants were induced by micronutrients, 2, 24 and 72 h before challenge with S. fuliginea. The level of protection induced by various concentrations varied from solution to solution. In general, the systemic protection induced by K2HPO4 was similar to that by the microelements. Spraying of a 1:1 mixture of phosphate and micronutrient solutions did not improve the systemic protection over that obtained with each of the solutions alone. Increasing the inoculum concentration of S. fuliginea increased the number of powdery mildew colonies produced on both induced and non-induced plants and has relatively affected the systemic protection on induced plants. A single foliar spray of micronutrient solutions, as a prophylactic treatment, on the upper surface of all the leaves of 3-leaf stage cucumber plants significantly inhibited powdery mildew development. A single spray of MnCl2 on leaf 1 elevated peroxidase activity in the soluble fraction and caused an enhancement of β-1,3-glucanase content in the ionically bound fractions of leaf 2 of non-inoculated plants. Forty-eight hours after inoculation, the level of both fractions of the enzymes increased in non-treated plants and decreased (β-1,3-glucanase) or remained unchanged (peroxidase) in treated (induced) plants as compared to non-treated plants. The possible mechanism for this protection, and the use of microelements and phosphate solutions as inducers for systemic protection and as agents for disease control are discussed.

Sensitivity of Red Delicious Apple Fruit at Various Phenologic Stages to Infection by Alternaria alternata and Moldy-core Control

The fungus Alternaria alternata, is considered to be the predominant fungus involved in moldy-core of Red Delicious strains of apple. In this paper, we report on the sensitivity of various phenologic stages to infection by A. alternata, and on the efficacy of various fungicides in controlling moldy-core disease in apple orchards. Artificial inoculations conducted in the orchard during 1999 and 2000 seasons revealed that the beginning of bloom (10–30%) and full bloom were the most susceptible developmental stages for infection. Natural infection with A. alternata in fruits was relatively high, reaching 44% and 46% of the fruits on control non-treated trees in 1999 and 2000. Four foliar applications of polyoxin B, difenoconazole and azoxystrobin, starting from the beginning of bloom until fruit set, reduced the number of infected fruits by 54–70%, 61–70% and 50–65%, respectively, compared with non-treated trees. Four or eight foliar applications of each fungicide provided similar levels of control. There were no significant differences between two, four or six foliar applications of difenoconazole, neither between two or four applications of polyoxin B. Adding CaCl2, as a tank mixture with difenoconazole at full rate, did not improve efficacy. Alternaria was recovered from the inner part of the core region of 71–88% of the fruits of the non-treated control, but was recovered less frequently from the outside part of the core region. Fruits of difenoconazole and polyoxin B treated trees were less colonized with A. alternata at both the inner and outside parts of the core region, as compared with controls. Results indicate that a control programme based on spray applications of difenoconazole or polyoxin B, during bloom period, can effectively reduce Alternaria on Red Delicious.