Plums and cherries, particularly those grown in the UK, are among the favourite fruit of consumers. New production techniques and new varieties of cherries have renewed interest in these crops amongst UK growers, with the opportunity for meeting more of the UK demand with home-produced fruit. However, fungal rots in the orchard and especially post harvest after short-term storage, may be the limiting factor in realising the full potential of these valuable lines. With funding from Defra research has been initiated to tackle the problem and develop effective solutions.

WHICH ROTS ARE IMPORTANT?

Generally, compared to apples and pears, UK-grown plums and cherries have not needed intensive fungicide programmes to combat disease problems. Blossom wilt and brown rot (Monilinia laxa) are usually considered the main problems affecting fruit, but no recent studies have been done to confirm that this is still the case. The first two years of the Defra project (2002-03) have therefore focused on identifying the main causes of rotting in orchards and post harvest. Regular visits were made to plum and cherry orchards in the main fruit growing areas of the UK (Kent, Herefordshire, Somerset, Cambridgeshire) throughout the season and observations made on rot incidence. At harvest random samples of fruit were collected from cherry orchards and stored at between 0°C and +2°C for up to four weeks. Samples were removed from store after one, two and four weeks and the rots assessed after seven days shelf life. Plum orchards were also sampled at harvest and stored at the same temperature for up to six weeks and the rots similarly assessed in samples removed from store after one, three and six weeks.

CHERRY ROTS

The studies were done on the varieties Stella and Colney, as these were present on most farms. The incidence of blossom wilt varied considerably between orchards and by variety. Blossom wilt was generally low in Colney orchards but in Stella orchards varied from zero-32 per cent of blossoms wilted. Laboratory tests on healthy green fruits, collected from orchards soon after blossom, indicated that up to 47 per cent of fruits were infected with Monilinia laxa, and a lower percentage with botrytis. This represents fungal infection that probably occurred during flowering, but will remain symptomless until the fruit start to mature i.e. near harvest or post harvest during marketing.

Despite the wet weather of 2002, the incidence of fruit rotting at harvest was generally low. Brown rot, caused mainly by Monilinia laxa (characterised by grey pustules on the rotted fruit) was the predominant rot present with up to 33 per cent rotted fruit recorded in the worst orchard. Brown rot caused by Monilinia fructigena (Buff pustules on the rotted fruit) was also present but at a much lower incidence. Cherries rotted with Mucor, Penicillium, Colletotrichum and cherry scab (Venturia cerasi) were also recorded in some orchards at harvest. Losses due to rots in Stella were very high (76 per cent) even after one week in store. Mean losses after one month in store were 86 per cent with losses of 100 per cent recorded for fruit from some orchards. Monilinia laxa, Monilinia fructigena and botrytis were the main rots identified. Losses in Colney were lower than in Stella, but still unacceptably high.

PLUM ROTS

The varieties Victoria and Marjorie's Seedling were selected for study, as these are generally the most widely grown. The incidence of blossom wilt varied widely between orchards and by variety. The disease was not seen in most of the Marjorie orchards, but in the Victoria orchards it varied from zero-10 per cent infected blossoms. Laboratory tests on healthy green fruits collected from the orchards soon after blossom indicated that while fruitlets from Marjorie's Seedling orchards were mainly disease-free, up to 98 per cent of fruits from Victoria orchards were infected with Monilinia laxa. This is a fungal infection that probably occurred during blossom but will remain symptomless until the fruit start to mature. At harvest, Marjorie's Seedling orchards were relatively free of rotting fruit, whereas up to 13 per cent of fruit were infected with brown rot, mainly Monilinia fructigena, in Victoria orchards. Rotting in Victoria was relatively high even after one week (48 per cent). Mean losses after six weeks were over 60 per cent. Brown rot caused by Monilinia laxa was the predominant rot. Less than 10 per cent of fruit of Marjorie's Seedling rotted, even after storage for six weeks.

Similar results for rots in plums and cherries were found in 2003.

The studies so far indicate a significant problem with rotting in cherries and Victoria plums, which must be solved. Most of this rotting is attributable to the brown rot fungi, particularly Monilinia laxa, which also causes blossom wilt in addition to brown rot. The variety Marjorie's Seedling appears to be much less susceptible to these problems and may be more suitable for short-term storage.

TACKLING THE ROTS

Monilinia laxa is difficult to control. The fungus appears to overwinter in cherry orchards mainly on mummified fruit and in plum orchards on mummified fruit and cankers. Further confirmation is needed of these observations. Mummified fruit produce a fresh crop of spores in December or even earlier. In spring, spores spread from the mummies in damp weather to infect flowers causing blossom wilt and reducing yield by killing flowers. Infected flowers then provide further inoculum for the developing fruits.

The key to effective fruit rot control lies in elimination of the overwintering inoculum and in the control of the blossom wilt phase of the disease. There are no products with label recommendations for control of blossom wilt and brown rot on plums and cherries. Indar (fenbuconazole), Systhane (myclobutanil) and Teldor (fenhexamid) have off-label approvals for use on plums and cherries and will give some control of the disease.

The most effective way of eliminating overwintering inoculum is hand removal of the mummified fruit during the autumn and winter before blossom. Where this is not practical, it may be possible to apply treatments to suppress sporulation on the mummies or cankers. Preliminary work has shown that in laboratory and field tests fungicides such as Indar (fenbuconazole), Folicur (tebuconazole), Rovral (iprodione) and Bavistin (carbendazim) almost completely suppressed sporulation on plum mummies. None of these products, apart from Indar (off-label approval), is cleared for use on plums or cherries.

This project is at an early stage, but it is clear that blossom wilt and brown rot are significant problems on cherries and on Victoria plums. The remaining work will concentrate on developing effective control measures for the early part of the season to avoid excessive fungicide use nearer harvest.

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