In this section:
Management of Stone Fruit Diseases
Principles of Control
In the main, fungi are the major disease organisms which cause problems in the production of stone fruit. When dealing with fungal diseases, protection is a better policy than eradication. Once a fungal disease becomes established, there are only a limited number of registered eradicant fungicides available and if the disease is widespread or advanced before detection and application, these chemicals often have limited beneficial effect. Even if the disease is controlled at this stage, considerable loss in yield or fruit quality is likely to have occurred.
Crops can be protected in two major approaches. Hygiene in orchards can not be over emphasised. This means the removal and disposal (burning or burying) of diseased wood and fruit. Careful hygiene can substantially reduce the incidence of disease in an orchard and will reduce the reliance on the second form of disease prevention, that is, protectant chemicals.
Spray timing is an essential part of good use of protectant fungicides. Well timed sprays can effectively control many diseases, but to be effective growers need to be familiar with the environmental conditions which favour disease infection and development. Also, the critical times at which stone fruit are most susceptible to attack is important. Without a sound knowledge of these factors for each disease, protection will not be efficient, nor effective.
Another factor which needs to be considered is the level of inoculum present in the orchard or surrounding orchards. For instance if the disease was noticed in the previous season it warrants close attention in the coming season.
Critical Times of Susceptibility
Bud burst
Young flower and leaf tissue is particularly susceptible to fungal infection. Hence spring is a critical time in which to prevent infection. The large amount of young susceptible leaf and flower tissue present provides an easy target for disease if spores are present and conditions are favourable. It is important that this tissue is protected if conditions are favourable for fungal infection. In dry seasons, fewer sprays will be needed than in wet seasons.
Visible symptoms of some diseases will not appear until 7–14 days after the time of infection, ie. the weather event that provided favourable warmth and wetness in the orchard. Fungal diseases can dramatically reduce tree health, yield and fruit quality if precautions are not taken from the time of budswell on. How much and for how long protection needs to be provided depends mainly on weather conditions.
Pruning
Eutypa dieback and silverleaf fungal diseases infect stone fruit trees through fresh wounds to the wood. Chances of bacterial canker infection are also enhanced by the presence of untreated cut surfaces. Making wounds is unavoidable when pruning, therefore it is essential that precautions to minimise the risk of infection are taken. Where possible pruning should be avoided during and prior to periods of moist or rainy weather. Cut surfaces should be protected with a fungicide solution, wound dressing or copper based paint, especially cuts to wood of greater than 10 millimetre diameter. The bigger the cut, the higher the risk of an airborne spore landing on it.
Consider summer pruning. Cuts at this time of growth heal more quickly and the weather is often drier. Summer pruning does not replace winter pruning, but will reduce the number and size of cuts which need to be made in winter.
Factors Determining Disease Incidence
Weather
Nearly all fungal diseases of stone fruit require moisture to germinate and subsequently infect. For this reason disease is most likely to occur after rainfall, especially if followed by prolonged periods of high humidity. Powdery mildew is an exception.
Thankfully, overhead irrigation, used on hot days, will wet the foliage but often not for long enough periods to provide sufficient humidity for disease to develop.
Temperature is also important; 15–25°C is often ideal.
In summary, if the critical spring period is dry, you can expect a low incidence of fungal diseases and few if any sprays will be needed. However it is usually worth applying at least one copper spray between early budswell and early pink bud to provide protection at this critical early season period.
Usually follow on sprays will only be needed in wet weather, so use your assessment of the prevailing weather to determine how many cover sprays you apply. Take care in wet springs.
Level of inoculum
Inoculum is the term for spores or other agents which cause disease. If a disease was present this season, it is likely that the disease pressure will be high in the following season. As a result, if conditions favour disease, onset of the disease will be rapid. If there are low levels of inoculum in the orchard, it takes far longer for the disease to reach economic levels of crop loss and damage. However it is good to prevent the build up of disease in such seasons to reduce the inoculum for subsequent seasons.
Disease carry over
Towards the close of a season where diseases have been prevalent, autumn copper sprays can have a significant impact on stone fruit diseases. Disease carryover refers to the over-wintering of diseases which remain present in the tree in a dormant state. When new seasons growth is present, and if conditions again favour disease development, the diseases become active and symptoms quickly appear. Diseases for which autumn sprays give benefit are shothole, freckle, stone fruit rust and to a lesser degree bacterial canker.
