HDC Factsheet 02/07 F.Indd

Factsheet 02/07 Horticultural Bradbourne House Development East Malling Cane fruit Council Kent ME19 6DZ T: 01732 848383 F: 01732 848498 E: [email protected] Phytophthora root rot of raspberry and other cane fruits by Scott Rafﬂe and Janet Allen, ADAS This factsheet provides information on the biology of Phytophthora fragariae var. rubi and the effect it has on raspberry and other cane fruit crops. It summarises techniques that should be employed both to avoid and contain infection and offers the currently approved control measures. Background being Phytophthora fragariae var. rubi uncommon for plantations to die out (P. fragariae var. rubi). The disease was completely within three years of initial Phytophthora root rot of raspberry and first reported in raspberry plantations in infection occurring. The disease led other cane fruit crops brings about Scotland in 1937. Further reports were to significant reductions in total yields root death, leading to die back of both made in North America in the 1950’s being produced by raspberry planta- the floricanes (fruiting canes) and pri- and 1960’s, but it wasn’t until the mid tions in Scotland in the 1980’s and mocanes. The root rot can be caused 1970’s that serious outbreaks began to 1990’s, and made a significant con- by a number of species of the soil- occur in the United Kingdom, leading tribution to the rapid decline of rasp- borne fungi Phytophthora. The most to significant loss of plantations in berry production in Scotland during important is now recognised as both Scotland and England. It is not that period (Figure 1). Photo supplied by SCRI 1 Phytophthora root rot has led to rapid die-back of raspberry plantations and a corresponding decline in raspberry production in Scotland Symptoms be found to possess very few, if any red/brown in colour, with a distinct mar- root hairs. If the epidermis is scraped gin at the interface between diseased Symptoms generally first appear on away from surviving main roots using and healthy tissue. Dead root tissue single plants or isolated groups of a pen knife, the underlying cortical tis- can sometimes be found as far back as canes within a plantation (Figure 2). sue is usually found to be necrotic and the crown region of plants. As the disease develops, it spreads to the roots of other canes, very often along the length of the crop row. Symptoms can develop on both primocanes and floricanes. Where plants are affected, fewer numbers of primocanes may emerge in the spring months compared to healthy plants and those that do may be weak. Infected primocanes may exhibit signs of wilting and die back shortly after emergence. The tips of the primocanes often wilt first, producing a characteristic ‘shepherd’s crook’ effect (Figure 3). Affected primocanes may initially develop water-soaked lesions on one side, which run upwards from their base. These later turn dark purple Photo supplied by SCRI or brown in colour as the canes wilt 3 Affected spawn exhibiting the classic 4 Water soaked lesions develop at the base and die (Figure 4). Where the die back ‘shepherd’s crook’ effect of spawn, which may later turn dark purple or of primocanes is slower, the leaves brown in colour gradually turn chlorotic, wilt and then die during the summer months. In some instances, primocanes may be infected but survive and remain symptomless until the following season, when they become floricanes. Affected floricanes produce weak fruiting laterals that tend to wilt or turn yellow and necrotic either around the leaf margin or between the leaf veins. Severely affected floricanes wilt and usually die before harvest begins. Affected plants or stools often have weak and poorly developed root systems when compared to healthy plants (Figures 5 & 6). When the roots of in- Photo supplied by SCRI fected plants are examined, they will 5 Illustration of poorly developed and dying root systems (three roots on left) compared to healthy roots (on far right) Photo supplied by SCRI 2 Symptoms ﬁrst appear on single plants or 6 Declining root volume in module raised plants caused by increasing levels of Phytophthora infection isolated groups of canes Infection and spread production of sporangia and discharge when soils begin to drain and dry out, of zoospores are favoured by moist and normally in the summer months. Initial infection and spread can be wet soils (Figure 7). Optimum discharge Early infections are often (but not brought about by several means: of zoospores occurs when soils are always) located in areas of a plantation • By planting infected plant material on saturated with water and at soil temper- where the drainage is impeded. They clean land or into soil less substrate atures of between 4–25oC. The most can also be found in areas where the soil common period of infection therefore has a clay texture or in low lying areas • The movement of contaminated soil occurs during periods of prolonged of the field to which soil water drains. or run-off water from infested to wet weather. In the United Kingdom, Extended periods of wet weather can clean sites such conditions most commonly occur spread the infection along the length between autumn and spring. The dis- of a row or to other low lying areas of • Use of contaminated irrigation water charge of the zoospores is impeded the plantation. The movement of contaminated soil on plants, farm machinery, footwear or clothing can rapidly lead to the spread of this disease around an individual or several farms. Conditions that favour spread The spread of the fungus P. fragariae var. rubi is dependent upon the presence of high levels of soil moisture. The most common form of inoculum is the zoospore stage. These zoospores are contained within sporangia. The 7 Waterlogging and poor drainage can lead to root death and Phytophthora infection Life cycle and a passive phase when the fungus soil from dead or dying root tissue. remains inactive in the soil (Diagram 1). These oospores can remain viable for The life cycle of P. fragariae var. rubi The fungus survives during this pas- many years in the absence of a host consists of two phases – an active phase sive phase in the form of dormant and are relatively insensitive to environ- when infection of raspberries occurs oospores, which are released into the mental extremes and fungicides Diagram 1 Life cycle of the fungus Phytophthora fragariae var. rubi •• •• •• • • • • Infected nursery plants •• • Sporangium High soil • • • •• • • • • • or symptomless plants • • •• moisture carrying the fungus Zoospores Active Phase Root infection Zoospore germination Mycelium in • •• and penetration of root • • • •• • • •• living tissue • • • • • • Oospore germinates Passive Phase Secondary infections within new roots • • • • •• • Movement of infested soil • • • • •• •• • • • (on clothing, machinery) • • • Oospore Overwinters as oospores • •• • or irrigation water • in soil or rotting plant • • contained in ••• •• an oogonium debris (including roots) applied to the soil. They can be moved In the active phase, sporangia are sections of roots being asphyxiated to new sites either on rotting roots or produced which release zoospores or root cracking. This in turn leads to plant debris, in soil, on clothing, during periods of wet soil conditions. an increased release of root exudates, machinery or in irrigation water. In the These move through soil water and to which zoospores are chemically presence of new host plants, the attack raspberry root tissue, setting attracted. The fungus can persist for oospores can germinate, giving rise up new infection. Prolonged periods some time as mycelium, within the to the active phase. of soil water logging can also lead to living plant tissue. Cultural control during the growing season. It must be material, infected soils or irrigation emphasised that no health scheme can water. Although peat and most other measures guarantee freedom from disease, but soil-less substrates cannot be they can help to reduce the risk. considered to be sterile, if manufac- Propagation tured, handled, stored and decanted into pots, bags or troughs correctly, Raspberry propagators must be partic- Site, soil and substrate at the point of planting they should not ularly vigilant to the risk that visitors selection contain P. fragariae var. rubi spores. pose to their units, who may have come To produce conditions that are not con- from infected premises. To reduce Selecting virgin land, which has never ducive to the development or spread the risk of cross contamination, when produced raspberries or cane fruit of this fungus, substrates should be visitors arrive on their premises, prop- crops before is the best way of avoiding chosen that have a high level of air agators should: infection, although the availability of filled porosity (preferably 15% or more), • always insist that clean protective such land is often scarce in the main to allow free drainage and promote footwear is worn that is fit for the fruit growing areas. Phytophthora healthy root development. purpose infection has become so common in raspberries in the UK, that sites that • always insist clean coveralls are worn have grown raspberries before should Organic matter and soil be considered to have a high risk of additives • only allow the propagation site containing the fungus. To completely vehicles to be used for transport avoid any risk of infection, it is better Before planting, some growers incor- around the property to plant raspberries and cane fruit crops porate organic matter such as well on virgin sites or in soil-less substrates rotted farm yard manure or composted than to use previously cropped soils. green waste. When used in heavy or Healthy planting material Where the only option is to plant in clay based soils, this helps to improve soil that has previously been used for soil texture, structure and drainage.

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