Maurizio Vurro

Emerging infectious diseases of crop plants in developing countries: impact on agriculture and socio-economic consequences

Emerging infectious diseases (EIDs) caused by plant pathogens can develop into unexpected and very serious epidemics, owing to the influence of various characteristics of the pathogen, host and environment. Devastating epidemics, having social implications by increasing the rate of urbanization, occurred in the past in Europe, and many other EIDs still occur with high frequency in developing countries. Although the ability to diagnose diseases and the technologies available for their control are far greater than in the past, EIDs are still able to cause tremendous crop losses, the economic and social impact of which, in developing countries, is often underestimated. In the present article, four of the most important EIDs in developing countries are considered from the standpoint of their origin, characteristics, symptoms, mode of spread, possible control strategies, economic impact and the socio-economic consequences of their dissemination. They are Cassava Mosaic Virus Disease, capable of reducing yields by 80–90% and causing the suspension of cassava cultivation in many areas of East Africa; Striga hermonthica, a parasitic weed affecting cereals in an area of at least 5 million hectares in Sub-Saharan Africa; Xanthomonas Wilt of Banana, a bacterial disease that caused around 50% yield losses at the beginning of 21st century in Uganda and is threatening the food security of about 70 million people owing to its impact on an important staple crop; and race Ug99 of the rust fungus Puccinia graminis f. sp. tritici, which is having a tremendous impact on wheat in Uganda, and is also threatening most of the wheat-growing countries of the world.

Recent advances in the biocontrol of Orobanche (broomrape) species

Parasitic broomrapes (Orobanche spp.) are majoruncontrolled weeds in the Mediterranean regions of Europe and the NearEast causing major losses to vegetable, grain legume, and sunflowercrops. Selective herbicides alone cannot provide persistent, season-longcontrol of these parasites, and much methyl bromide is used for theircontrol, where affordable. Thus they are excellent targets forbiocontrol. The recent progress by the COST 816 Orobancheworking group in this area is reviewed herein. Natural infestation bythe fly Phytomyza orobanchia of seed capsules of Orobanchecrenata parasitising faba bean halved Orobanche seedproduction while inundative releases of adults reduced it to 5%of viable seeds. The fungi Fusarium arthrosporioides E4a andF. oxysporum E1d, as well as strains of bacteria were isolatedfrom diseased, juvenile, Orobanche flower stalks. They arepathogenic to O. aegyptiaca, O. crenata and O. ramosaon most vegetable crops. A F. oxysporum f. sp.orthoceras was specifically pathogenic to O. cumana onsunflowers. All were used in various experiments with a modicum ofsuccess. Methods were developed to formulate isolated mycelia, whichcould eventually allow the use of transgenic hypervirulent pathogens inasporogenic (deletion) mutants (as a failsafe against spread).Mycotoxins were also isolated from different Fusarium and otherfungal species that kill Orobanche, and are being consideredfor direct use, or to augment other strategies. All threeFusarium spp. used have been transformed with gusand/or gfp genes allowing tracing their movement in theenvironment, and opening the way to future transformations tohypervirulence.

Fusaric and 9,10-dehydrofusaric acids and their methyl esters from Fusarium nygamai

Abstract Fusaric and 9,10-dehydrofusaric acids and their corresponding methyl esters were isolated from the culture filtrates of Fusarium nygamai . The methyl esters were characterized by chemical and spectroscopic methods and reported here for the first time as naturally occurring products. When assayed on tomato leaves and seedlings at 2.7 × 10 −3 and 2 × 10 −4 M, respectively, fusaric and 9,10-dehydrofusaric acids and their methyl esters showed wide chlorosis rapidly evolving into necrosis as well as a strong inhibition of root elongation, respectively. When assayed at 10 −4 M on brine shrimps ( Artemia salina ), fusaric and 9,10-dehydrofusaric acids did not prove to be toxic, while their methyl esters showed a toxicity level of 50%, expressed as mortality.

Cytochalasins: structure-activity relationships

Abstract Seven cytochalasins, 17- O -acetylcytochalasin A and two derivatives of cytochalasin B were assayed for Pseudomonas syringae , Bacillus megaterium , and for Geotrichum candidum . Their ability to inhibit the growth of tomato seedlings and their toxicity to brine shrimp (% larvae mortality were also investigated. Among the compounds assayed only cytochalasin A showed antibiotic and fungicidal activity. Toxicity to tomato seedlings was exhibited by both [14]- and [11]-macrocyclic cytochalasans, while the activity decreased in the derivatives acetylated on the 7-hydroxy group and markedly in cytochalasin E. In the brine shrimp bioassay, cytochalasin E was the most active mycotoxin, but generally, at low concentrations, the cytochalasins with the [11]-macrocyclic ring were more active than those with the [14]-macrocyclic ring; cytochalasin A appeared to be the most toxic.

Pinolidoxin, a phytotoxic nonenolide from Ascochyta pinodes

Abstract Ascochyta pinodes , the causal agent of pea anthracnose, cultured on sterilized wheat, produces toxic metabolites. The main phytotoxin, named pinolidoxin, was isolated and characterized using spectral and chemical methods as 2-(2,4-hexadienoyloxy)-7,8-dihydroxy-9-propyl-5-nonen-9-olide, a new phytotoxic 10-macrolide. When assayed on host (pea) and non-host (bean) plants, pinolidoxin was highly toxic; towards brine shrimps it was only weakly toxic.

Cytochalasins Z1, Z2 and Z3, three 24-oxa[14]cytochalasans produced by Pyrenophora semeniperda

Three new cytochalasans, named cytochalasins Z1, Z2 and Z3, were isolated from the wheat culture of Pyrenophora semeniperda, a fungus proposed to biologically control grass weeds. Other cytochalasins isolated from the same organic extract were identified as the already known cytochalasins F, T, deoxaphomin and cytochalasins B, the latter being produced in very large amounts. All three new cytochalasins were characterized as 24-oxa[14]cytochalasans by extensive use of NMR and MS techniques. Cytochalasins Z1 and Z2 proved to be structurally related to cytochalasin T, whereas cytochalasin Z3 was related to cytochalasin B. When assayed on wheat and tomato seedlings, cytochalasin Z3, in comparison to the new cytochalasins, cytochalasin B, its 21,22-dihydroderivative, cytochalasin F and deoxaphomin showed a remarkable ability to inhibit root elongation. The possibility of using these metabolites in biological control strategies is discussed.

Brefeldin A and α,β-dehydrocurvularin, two phytotoxins from Alternaria zinniae, a biocontrol agent of Xanthium occidentale

Abstract Alternaria zinniae, a fungus causing leaf necrosis in Xanthium occidentale, a widespread noxious weed of Australian summer crops and pastures, produces toxic metabolites when grown in liquid culture. Extensive application of NMR techniques, HR, EIMS and X-ray analysis permitted the identification of brefeldin A (4H-cyclopent[f]oxacyclotridecin-4-one-1,6,7,8,9,11a,12,13,14,14a-decahydro-1,13-dihydroxy-6-methyl) and α,β-dehydrocurvularin (4,5,6,7-tetrahydro-11,13-dihydroxy-4-methyl-2H-3-benzoxacyclododecin-2,10 (1H)-dione), two bioactive metabolites produced by a number of fungal species belonging to the genera Alternaria, Ascochyta, Penicillium, Curvularia, Cercospora, and Phyllosticta. This is the first report on the isolation of these two phytotoxins from A. zinniae. The interesting semi-selective toxicity to X. occidentale is described here, and the possible use of fungus and toxins in integrated weed management programs is discussed.

Putaminoxin, a phytotoxic nonenolide from Phoma putaminum

Phoma putaminum, the causal agent of leaf necrosis of Erigeron annuus, a common weed of field and pasture, produced toxic metabolites when grown in liquid culture. The main phytotoxin, named putaminoxin, was isolated and characterized using spectroscopic and chemical methods as (5S)5-hydroxy-9-propyl-6-nonen-9-olide, a new 10-macrolide. When assayed on leaves of host and non-host plants, putaminoxin showed a wide range of toxicity, with leaves of E. annuus being most sensitive.

Trans-4-aminoproline, a phytotoxic metabolite with herbicidal activity produced by Ascochyta caulina.

A phytotoxic metabolite, characterized through NMR techniques and synthetic methods as trans-4-aminoproline, was isolated from the culture filtrates of Ascochyta caulina, a promising mycoherbicide for biological control of Chenopodium album. The metabolite, which shows interesting phytotoxic properties, together with ascaulitoxin (recently characterized as N.2-beta-D-glucoside of the unusual bis-amino acid 2,4,7-triamino-5-hydroxyoctandioc acid) and another unidentified compound, compose an active fraction of A. caulina culture filtrates with promising herbicidal properties. When assayed on leaves of host and non host dicots, including wild and cultivated plants, the trans-4-aminoproline showed a wide range of toxicity, with leaves of C. album being the most sensitive. Other interesting aspects were its inefficacy on several monocots, both cultivated and wild, and its lack of antifungal, antibiotic and zootoxic activities. This is the first report on trans-4-aminoproline as naturally occurring compound and phytotoxic metabolite produced by A. caulina.

Natural metabolites for parasitic weed management

Compounds of natural origin, such as phytotoxins produced by fungi or natural amino acids, could be used in parasitic weed management strategies by interfering with the early growth stages of the parasites. These metabolites could inhibit seed germination or germ tube elongation, so preventing attachment to the host plant, or, conversely, stimulate seed germination in the absence of the host, contributing to a reduction in the parasite seed bank. Some of the fungal metabolites assayed were very active even at very low concentrations, such as some macrocyclic trichothecenes, which at 0.1 microM strongly suppressed the germination of Orobanche ramosa L. seeds. Interesting results were also obtained with some novel toxins, such as phyllostictine A, highly active in reducing germ tube elongation and seed germination both of O. ramosa and of Cuscuta campestris Yuncker. Among the amino acids tested, methionine and arginine were particularly interesting, as they were able to suppress seed germination at concentrations lower than 1 mM. Some of the fungal metabolites tested were also able to stimulate the germination of O. ramosa seeds. The major findings in this research field are described and discussed.