Mid-summer and everything is growing well… or it should be. There are a number of problems which may become apparent at this time of year, some of which can especially be a problem for newly planted trees and shrubs, or for those growing in containers for that matter, so here’s a quick list of some of the ones our customers quite often encounter, a little bit about them, and how to deal with them:-
Vine weevil rarely cause major problems for established trees and shrubs as, in healthy soil with an established ecosystem, the grubs which do the damage are predated by ground beetles, centipedes and so on, limiting populations enough that healthy, established specimens can just shrug off the limited amount of root damage. Vine Weevil grubs can sometimes cause problems for newly-planted trees and shrubs while they’re battling to get established – but the biggest area of concern is for plants growing in containers where the grubs can feed in large numbers, safe from anything that might eat them.
What to look out for: irregular notching on leaves is usually a good sign that adult weevil have been active and feeding and therefore there are likely to be, or soon will be, grubs feeding on the roots. For this reason we place pots of Bergenia cordifolia, which is a semi-evergreen large-leafed perennial on which any notching is really easy to spot, in amongst our container stock on the nursery as an indicator to check for any adult weevil activity. The grubs are a particular problem in light, open growing media such as soil-less composts (ie in peat-based or peat-substitute composts). For some softer plants, such as strawberries or Heuchera, the roots may be eaten-off completely, but for woody plants such as shrubs the damage may appear more subtle as the grubs can’t eat the woody roots but can still cause the failure of the entire plant if populations are high enough by eating the finer roots and by nibbling the softer outsides of the roots.
So what are Vine Weevil? Weevils look like beetles but don’t have any wings, so at least they don’t fly in… but their march can be seemingly relentless and you’d be surprised what they can climb! The adults avoid light and are mostly active at night so you can go hunting for them after dark from spring to autumn as they will climb up from their daytime hiding place (somewhere dark and protected) to feed on leaves at night. Placing sheets of newspaper under/around a suspected infested shrub just before giving it a good shake/brush at night is a good way to disturb and knock off adults, which you can then just squish. However, this will only limit populations and, since a single adult can lay hundreds of eggs each year, it’d be safe to assume that any signs of notching at all means that there will be some grubs. Since there aren’t any effective or environmentally safe/responsible systemic insecticides available anymore which can be pre-mixed into the compost for container stock on the nursery we treat our stock by watering with nematodes (microscopic parasitic/pathogenic eelworms); this is the only effective treatment available to the public also. These need to be applied when soil/compost temperatures are relatively warm.
Photo: Bergenia cordifolia are placed around our nursery as an indicator plant – thankfully there’s no notching on the leaves so it’s safe to assume that there are no Vine Weevil active here!
Scab diseases cause dark spots on fruit and leaves, as well as blistering or cracking on badly infected twigs. Apple Scab affects Malus trees, whether as edible apple trees or ornamental Crab Apples, Pyracantha, Cotoneaster and occasionally Sorbus (eg Rowans), whereas Pear Scab only affects Pyrus (edible and/or ornamental Pear trees). Scab diseases can overwinter on fallen leaves or infected twigs, but initial infections are commonly airborne. Infections spread more rapidly during the growing season by splash between leaves so cases are far worse in wet summers. On fruit trees the scab affects the quality of the fruit, making it less attractive but is often only skin deep and therefore, if the skin isn’t cracked allowing in secondary rots, the fruit can still be eaten. On ornamental trees a scab infection is largely cosmetic but it can affect the vigour of the plant so maintaining otherwise good plant health to encourage growth is often enough to help them grow through. On the nursery we can spray against scab but this is rarely practical in a domestic garden as, even if you can find a suitable fungicide that might have some effect, spraying a large tree (ie overhead) is likely to result in you getting more spray on yourself than where it needs to be on the tree! Therefore, good hygiene in disposing of affected fallen leaves will certainly help, as will improving air flow around the foliage.
The powdery, dusty-white growth of Powdery Mildew diseases are a common sight on a wide range of plant species, although each specific disease is usually specific to a relatively limited range of plant species (eg the Powdery Mildew which affects cucumbers isn’t the same one that affects apple trees, pears, laurels, roses, etc). Like many fungal diseases, Powdery Mildew can overwinter on fallen leaves or on infected twigs, so good hygiene is important, as is maintaining good airflow around the foliage. However, Powdery Mildew is readily spread on air currents so most infections tend to be airborne and, although it thrives in humid conditions, doesn’t need the leaves to actually be wet – in fact it tends to be more common in drier weather rather than the wet summers that encourage diseases like Scab. It wouldn’t be a stretch to consider Powdery Mildew as an opportunistic disease as it readily infects plants which are already weakened and under stress, such as those suffering from drought or nutrient deficiencies. The fungal growth as fairly superficial on the leaf surface so is quite easy to treat with fungicides (but we don’t recommend spraying overhead at home as you’ll get it all over yourself!). Therefore, the best way to control Powdery Mildew is by prevention through good airflow, hygiene, regular watering of establishing plants (and sometimes for more established plants in dry weather), appropriate feeding, and so on.
Photo: Powdery Mildew on the leaves of an Ornamental Pear tree
Leaf diseases on laurels (various)
Cherry Laurel and Portuguese Laurel are our best selling plants and, as such, we tend to get the same enquiries and concerns from customers every year. After watering and feeding issues, we are asked about diseases affecting laurels, which tend to be those opportunistic conditions as, with all the best intentions from gardeners, plants are much more prone to stress while they are trying to establish in their new home than a few years down the line.
There are of course leaf spot diseases which affect laurels but these don’t tend to be much of an issue with establishing plants for our customers. However, as an airborne disease, Powdery Mildew (see above) can become an issue after planting… but in advanced infections it can look very similar to another opportunistic disease, bacterial Shothole. Powdery Mildew on laurels may show the usual, visible powdery fungal growth but more often what is noticeable is a puckered distortion of the leaves, accompanied by irregular yellowing and, as tissues eventually become damaged, can lead to necrotic brown patches which die and drop out, leaving wiggly dead edges and holes.
Shothole also leads to small patches on the leaves dying and dropping out, leaving holes. It is common to members of the cherry family and, like Powdery Mildew, it mostly affects plants that are already stressed, sometimes after a mildew infection, but often when plants are hungry for nutrients (especially in spring on laurels growing in containers). There are no effective chemical controls for Shothole available to gardeners but it is so much an opportunistic disease that we tend to think of it more as a physiological disorder as, if the weakening growing conditions are improved (and badly affected leaves can be picked off if you wish), the plant will usually just perk up and grow through it once vigour has returned. Therefore, as well as regular and correct watering to maintain effective growth on laurels (and water directly to the roots and not to the leaves – don’t use sprinklers unless absolutely necessary), reduce competition from grass and nearby growth where possible. Establishing laurels and Portuguese Laurels are hungry feeders so we recommend giving them a good feed with a high-nitrogen plant food (and also Epsom Salts for Magnesium) in spring, and again in mid-summer, for the first two or three years during establishment… by encouraging strong growth this will help to fend off Shothole. However, don’t keep feeding with high nitrogen feeds into the autumn as this will encourage soft growth which will be more sensitive to frosts and will also be more vulnerable to Powdery Mildew.