Tomatoes are a major global food staple butPhytophthora infestans (an Oomycete) causes lateblight, a devastating disease that precludes commercial tomato production from moist temperate areas such as the United Kingdom and Northern Europe. We dissected the genetic architecture of resistance to lateblight as well as traits that improve yield and fruit quality in a tomato cross between a popular breeding, line NC 2 CELBR, which produces large fruits, and an heirloom cultivar called ‘Koralik’ which produces small, sweet fruits. We used an F2 mapping population to identify quantitative trait loci (QTL) for phenotypes including number of fruits, size of fruits, total crop yield, and soluble solids content in two different environments. Surprisingly, we found very few QTLs shared between the two environments, underscoring the importance of the local environment and genotypeby-environment interactions. We also assayed the virulence of three different isolates of P. infestans to identify QTLs that confer some resistance to the pathogen. We found nine crop-related QTLs and two QTLs for late-blight resistance-related phenotypes. One late-blight resistance QTL was inherited from Koralik (Chromosome 11, 70.2–83.5 cM) and it probably represents an undiscovered source of late-blight resistance. Yield QTLs were also located on chromosome 11 where Koralik alleles increase fruit number and yield, and adjacent regions decrease fruit size. On Chromosome 9, Koralik alleles increase fruit sweetness (Brix) by 25%. These results indicate that Koralik is a valuable donor parent that can be used by tomato breeders in targeted breeding strategies for fresh market tomatoes.

An investigation has been made of selffertile (homothallic) progeny which are frequently encountered in matings of heterothallic Phytophthora species. The pattern of segregation of self-sterility from these homothallic isolates during vegetative growth, asexual and sexual reproduction has been studied in some detail.

ABSTRACT Two field experiments were conducted to study the effect of overhead sprinkling irrigation on oospore formation by the late blight fungus Phytophthora infestans in potato. Total rain (natural + sprinkling) accumulated in treatments of experiment 1 (winter 1997 to 1998) were 765, 287, and 219 mm and treatments of experiment 2 (winter 1999 to 2000) were 641, 193, and 129 mm. Sporangia from 11 isolates of P. infestans were combined in eight pairs, seven of A1 and A2 and one of A2 and A2 mating type, and were sprayed on field-grown potato crops (42 plants per plot at 7 m(2) each) and examined for their ability to form oospores in the host tissues. In experiment 1, oospores were recorded in a total of 132 of 1,680 leaflets (7.9%), 24 of 105 stems, and 2 of 90 tubers. In experiment 2, oospores were recorded in 40 of 519 leaflets (7.7%), but not in any of the 90 stems or the 45 tubers examined. Both the proportion of leaflets containing oospores and the number of oospores per leaflet increased with time after inoculation and were dependent on the rain regime, the position of leaves on the plant, and the isolate pair combination. In both field trials, increasing the rainfall significantly enhanced oospore production in leaves. Leaf samples collected from the soil surface had significantly more oospores than those collected from the midcanopy. Two pairs in experiment 1 were more fertile than the others, whereas the pair used in experiment 2 was the least fertile. The total number of oospores per leaflet usually ranged from 10 to 100 in experiment 1, but only from 2 to 10 in experiment 2. Maximal oospore counts in the field were 200 and 50 in experiments 1 and 2, respectively, but ranged from approximately 2,000 to 12,000 oospores per leaflet in detached leaves in the laboratory. We concluded that P. infestans can produce oospores in the foliage of field-grown potato crops, especially when kept wet by regular overhead sprinkling irrigation, but production was far below that in the laboratory.

ABSTRACT Four pairs of primers were designed for PCR amplification of known polymorphic regions of the mitochondrial genome of Phytophthora infestans. Digestion of the amplified products with restriction enzymes allows identification of previously identified haplotypes. Product P2 cut with MspI uniquely identifies haplotypes Ib and IIa, while types Ia and IIb are differentiated by digestion of product P4 with EcoRI. Digestion of products P1 and P3 gave results similar to that with digestion of P4, but amplification of these products was less robust. Thus, all four common haplotypes are identified by amplifying and digesting products P2 and P4. Identification of haplotypes was also possible from DNA extracted directly from small, late-blight lesions on both tomato and potato leaves, making isolation of the fungus unnecessary. A rapid and efficient method of monitoring changes in the pathogen population is facilitated. These PCR primers were also useful for differentiating other Phytophthora species.

Of all the pathogens of economically important plant species, Phytophthora infestans (Mont.) de Bary can rightly claim its place as the most notorious. Its occurrence in Ireland during the last century caused both the potato famine of 1846-49 and the establishment ofplant pathology as a scientific discipline by Rev. M. J. Berkeley and others. Even today, late blight frequently decimates potato crops during wet summers, despite significant advances in disease prediction, crop protection (usually with phenyl amide or dithiocarbamate fungicides) and the introduction of more resistant cultivars. Although' late blight has traditionally been associated with potatoes, in tropical climates with periods of high rainfall the same pathogen can be a significant factor limiting yields from tomato crops. In many developing countries, tomato represents an important food crop, not only because of the high price it commands compared to staples such as rice, but also because it provides a valuable source of vitamins when fresh and improves the flavour of meals when cooked. The requirement of tomatoes for moderately low night-time temperatures for efficient fruit-set limits production in the lowland tropics to the cooler seasons. Consequently summer production is centred upon cooler upland areas (e.g. in the Cameron Highlands of Malaysia and the Central African massif), where high rainfall levels are optimal for late blight infection. In such areas, late blight represents a serious problem, since application of costly imported fungicide is often ineffective due to dilution by rainfall and the potential occurrence offungicide-resistant strains of the pathogen. Research at the University of Wales, Bangor being funded by the UK Government Overseas Development Administration and in collaboration with the Asian Vegetable Research and Development Centre (AVRDC) in Taiwan has two objectives. Firstly, we are identifying promising sources of resistance to late blight by screening material from tomato germplasm collections; promising accessions are currently being field-tested in Taiwan, Tanzania and Costa Rica prior to the initiation of a breeding programme. Secondly, we are examining levels of genetic variability within populations of the pathogen from tropical areas; it is vital to know the extent of geographical variation in the aggressiveness and virulence within pathogen populations to maximize the possibility that resistance factors introduced into planting material will provide durable disease protection over a wide geographical range. An important component of our research therefore involves the acquisition of samples of the pathogen from a range of countries, both temperate and tropical. These will permit testing the resistance of promising tomato accessions against pathogens from the countries where the resistant cultivars will ultimately be grown. A further reason for wishing to assess P. infestans populations is the increase over recent decades in the international potato trade. The concomitant migration of the fungus in diseased tubers is believed to have led to significant population changes. We hope that by obtaining isolates ofthe fungus from around the world it will be possible to more accurately monitor these changes. Wherever tomatoes are grown without irrigation and without heavy use of fungicides, it is likely that late blight is present unless conditions are very dry. The symptoms include blackening of the leaves (Fig Ia; often spreading lesions originating from the tip of the leaf) and death of whole branches of the plant (Fig Ib). Infected green fruit become marbled brown in appearance (Fig Ic) without becoming soft (not to be confused with nutritional disorders such as blossom end rot, which usually affect the area around the tip of the fruit). Sporulation of the fungus (only under

Mature (three-week) and immature (five-day) sexual progeny were raised from two matings between isolates of Phytophthora infestans with 2C-DNA content (and therefore presumed diploids) but which differed for isozyme genotype and restriction fragment length polymorphisms (RFLPs) in their mitochondrial DNA (mtDNA) and nuclear DNA (nDNA). Approximately 90% of the progeny which were derived from mature oospores, but approximately 50% of the progeny from immature oospores, were classed as hybrids on the basis of their Gpi 1 genotype. All single-oospore cultures, whether derived from a mature or an immature oospore, contained a single mtDNA type from either the Al or the A2 parent. All single-zoospore lines derived from a single germ sporangium of a germinating hybrid oospore also had the same mtDNA type, Gpi 1 genotype and mating type. These data suggest that mtDNA is inherited uniparentally in P. infestans with no evidence for segregation, elimination or recombination of types. Mating type segregated differently in hybrid offspring; those with type II mtDNA (from the Al parent) had a preponderance of Al mating type. Some nRFLPs also segregated unexpectedly in the population with type II mtDNA, suggesting that any bias in the segregation of mating type is caused by viability disturbance rather than linkage between mating type gene(s) and mtDNA.

SUMMARYRelative DNA contents of samples of nuclei from hyphal tips and from gametangia at different stages of development have been obtained by cytofluorimetry. The occurrence of nuclei with 4C, 2C and 1C DNA complements in gametangia and mainly 2C complements in hyphae indicates that meiosis takes place in the gametangia of Phytophthora drechsleri.