Epidemiological and physiological studies of the effects of peach rosette and decline disease on the peach, prunus persicae L. Batsch

The incidence in the field of the disease peach rosette and decline (PRD), which is of considerable economic importance in the Goulburn Valley, causing fruit loss and tree death, was shown to increase from 0.9 to 91.3% in an orchard of cv. Golden Queen in 10 years. Similar results were found with the cv. Pullars Cling, in which infection increased from 1.5 to 29.7% over five years. The pattern of spread was mainly from infected trees to contiguous uninfected trees. This is consistent with the view that the main causal agent, prune dwarf virus (PDV), is transmitted only via the transfer of infected pollen : a previous finding in cherries which was confirmed in peaches. Prunus necrotic ringspot virus (PNRV) is the other virus always present in the field in PRD-infected trees. The mode of spread of PNRV is also by pollen. Within the tree, PDV moved erratically from the first infected limb, via phloem but not xylem, into the other limbs well in advance of the appearance of symptoms. Three months after flowering, PDV was detected in 65% of main limbs adjacent to the first infected limbs but in only 30% of limbs more remotely positioned on the tree. However, removing infected limbs within four weeks of flowering, when the initial infection was presumed to have occurred, did not prevent the movement of PDV into the rest of the tree. Laterals from peach trees infected with PRD were tested for the presence of PDV, using woody virus indicators (cvs Golden Queen, Italian Prune and Elberta seedling). Golden Queen was found to be a more reliable indicator for detecting PDV than Italian prune, as the presence of PNRV with PDV killed 71% of the Italian prune buds compared to only 34% of the Golden Queen buds. Golden Queen also developed more obvious foliage symptoms of PDV infection than Elberta seedlings. The probability of failing to detect PDV in infected field trees, using all three indicator plants, was higher in the first year of infection. The rate of spread of PRD was reduced in the orchard by preventing infected trees from flowering, either by removing obviously infected trees or by deblossoming. Removing infected trees resulted in a three-fold reduction in the spread of the disease in two seasons. Removing the flowers from infected trees before pollination reduced the spread of the disease by about half. This, only partial, control of the spread of PRD by tree removal or deblossoming was attributed to the presence of up to 14.3% of trees without symptoms being latently infected with PDV. It was observed that deliberate infection with PDV by pollen also resulted in a slow expression of the symptoms of PRD. The effects of PRD on the growth of young peach trees was obvious in the first three months of growth. There were considerable varietal differences in the severity of this effect. Those varieties based on cvs. Golden Queen or Levis Cling were more severely affected than the variety Elberta. The results from shoot elongation measurements agreed with those obtained from conventional growth analysis methods. These latter experiments showed that, after three months, the dry weight and leaf area of infected Golden Queen plants were reduced by 94%. The fruit yield from mature PRD-free trees was three times that of trees infected for the first season, even though symptoms were apparent only on one limb; and six times that from chronically affected trees infected for two seasons. The effect of virus infection on the photosynthetic ability of single, attached peach leaves was studied under laboratory conditions using infra red gas analysis. The constants derived from the equations describing the relationship between net photosynthesis (Pn) and both irradiance and CO2 concentration were used to analyse the effects of infection by PRD on photosynthetic characteristics of the leaf. The asymptotic value of Pn (Pmax) in young leaves was reduced 15% by PRD-infection, mainly through an increase in the "residual resistance" to 002 diffusion and a decrease of 23% in the parameter indicating photochemical efficiency. There was also evidence that the gas phase resistance was higher in infected leaves at low levels of irradiance. Dark respiration was 51% higher in infected leaves, but this difference was not significant. PRD did not reduce Pn in 60-day-old leaves, normal leaf senescence having a predominant and greater effect. It was concluded that PRD infection had its large effects on growth via a reduction in leaf area; the effects on the photosynthetic capacity per unit leaf area being minor. An effect of PRD infection on the translocation of 14C-assimilatesout of leaves was also observed. Infected leaves retained twice the assimilates than did uninfected leaves. It is concluded that the most promising methods of control of PRD include removal of infected trees, deblossoming suspected infected trees until diagnosis is confirmed, use of virus-tested plants, the gradual destruction of infected orchards and protecting young, healthy orchards from infection either by barrier crops or deblossoming the young plants until they reach an economic bearing age