Flint and Van den Bosh (1981) define a pest as any organism that diminishes the accessibility, quality, or value of a human resource, such as crop plants. This report specifically focuses on pests with respiratory abilities and mobility. Two specific organisms within the pest category will be analyzed: Greenhouse White Fly and Woolly Aphid. Furthermore, horticultural diseases discussed in this report refer to less active ailments that bear similarities to viruses and growths.
Within the category of diseases, I have chosen to research Botrytis and Apple Scab. One pest that will be discussed is the Glasshouse white fly (Trialeurodes vaporariorum), commonly referred to as the greenhouse whitefly. This particular pest poses a significant problem for various fruit, vegetable, and ornamental crops. It can often be found in glasshouses and other protected horticultural environments. The white flies are small sap-feeding insects with a length of approximately 2mm when fully grown. They possess four wings covered in wax and typically reside on the undersides of leaves. If heavily infested, these pests will take flight together in a small cloud when disturbed.
Infestations can have negative impacts on plants, as documented by The Canadian Journal of Plant Pathology. These impacts include hindering the plants from reaching their maximum potential and resulting in decreased vigor and yields. One example is the potential 25% decrease in yield for a tomato crop if left untreated. White flies typically lay around 200 eggs at once on the undersides of leaves, which then hatch into small nymphs that release a sticky substance called “honeydew” while feeding. This honeydew falls onto lower leaves and forms a black sticky substance similar to soot. Under favorable conditions, white flies are capable of continuous reproduction throughout the year.
The duration of development from egg to adult for Glasshouse whitefly is approximately three weeks at 70 degrees or four weeks at 60 degrees (Buczacki, Harris 1998). While these pests are not usually a problem outdoors due to their preference for warmth, they can be a significant issue for house plants and indoor glasshouse growers. To effectively control Glasshouse whitefly, it is important to maintain cleanliness and clear areas, including the surroundings of the glasshouse during summer where whitefly can utilize weed foliage for their reproductive cycle. In the glass house, common physical control methods involve using simple yellow sticky paper traps placed just above the plants to catch the adults. Biological control can be achieved through the use of a tiny wasp parasite known as Encarisa Formosa, but it requires warm conditions and time to establish its population. Insecticidal soaps can also be utilized to control whitefly numbers while not harming Encarsia formosa. However, other pesticides such as permethrin, bifenthrin, primiphos-methyl, and pyrethrum are hazardous to both humans and beneficial parasitic wasps and should only be employed when whitefly populations have become uncontrollable. It is recommended to apply these pesticides at five-day intervals.
Greenwood P and Halstead A (1997) Woolly Aphid (Eriosoma lanigerum) This aphid is a significant pest to apple growers and orchard owners in the UK. It attacks a number of woody plants, including ornamental Malus and Pyracantha. Unlike most other aphids, woolly aphids suck sap from woody stems rather than the foliage. They are blackish brown in colour and cause galling of the wood. Although they are not usually very damaging to more mature trees, they pose a serious problem for young saplings. Woolly aphids are particularly fond of attacking the thinner bark areas on trees.
Numbers are a problem in both new and old orchards as they can easily enter through freshly pruned branches or cracks in the bark. The woolly aphids feed on the bark, causing soft and lumpy growths on affected shoots. These shoots are visible during winter pruning. The young aphids find shelter in the bark and become active in March and April, secreting a waxy wool material. This material can contaminate foliage and fruit, causing inconvenience during harvest time in late August.
The substance offers protection against predators while the aphids, in small groups, consume young leaf buds and shoots. Infestations are primarily spread by crawling from one tree to another or can be blown from one tree to another under the right conditions. Alford D 2000 mentions that some winged forms appear in July, but they are usually rare. Multiple hatching events of flightless aphids occur during the summer. Young aphids seek shelter lower down in cracks of bark or near the ground, taking cover under the roots and surrounding foliage.
In autumn, the old adults of the Woolly Aphid become dormant until the following spring, dying out as winter approaches. Control measures for Woolly Aphid include physical treatments on fruit trees. During early to mid-winter when the plants are dormant, a tar oil wash can be applied. In summer, Malathion can be used against the young nymphs. Another environmentally friendly approach is to wrap grease bands around affected trees. These bands trap the grubs as they descend the trees in search of hibernation spots near the base or on the ground in autumn. The bands can then be removed in November and burned.
In the 1920’s, a Biological control called Aphelinus, a small parasitic wasp, was introduced in southern England to control Woolly Aphids. If these wasps are present in the aphid colonies, it is recommended to carefully limit the use of pesticides to avoid damaging the wasp populations (Buczacki, Harris1998). Despite limited empirical information on the extent of damage caused by Woolly Aphids, they pose a threat to the overall health and vitality of fruit trees, particularly young ones. The damage inflicted by Woolly Aphids can lead to cankers and result in poor crop yields.
Ultimately, the decrease in production would necessitate either replacing affected trees or removing damaged branches. Diseases such as Botrytis are caused by various infectious agents, including fungi, bacteria, and ‘virus-like’ organisms. These disease-causing agents are referred to as pathogens. Fungi consist of over 100,000 species and acquire nutrients by externally digesting material, distinguishing them from plants. They secrete enzymes into the material and subsequently absorb and digest the mixture.
Most fungi have the ability to produce both sexual and asexual spores, allowing them to rapidly multiply under favorable conditions. Pathogens can develop in various ways, with some only causing minor localized issues like spots on leaves. In contrast, others can completely cover and decay plant material, leading to the destruction of the entire plant, its fruits, and seeds. Botrytinia, commonly known as botrytis, is a highly destructive grey mold fungus that poses a significant problem for soft fruit growers and vineyards worldwide.
Botrytis spores are always present in the air and can survive for up to 30 weeks in dead plant tissues. These spores form black resting structures called sclerotia, which help carry the fungus during periods when there are limited living host plants. If certain conditions are met, such as plant damage or damp and humid weather, the spores can attack and infect live plants, particularly their fruits and flowers (Philip, A etal 2007).
Botrytis, also known as grey mould, is a type of fungi that can invade plant flowers and remain inactive until the fruits start to ripen. When the sugar levels increase during the ripening stage, the fungi can quickly reproduce and create a grey mould on the surface of the fruit. Botrytis flourishes in warm and humid environments typically found in glass houses and polytunnels. To prevent its spread, it is crucial to ensure adequate ventilation in enclosed growing areas. One prevalent variety of botrytis is called botrytis cinerea.
Botrytis, a fungal disease, thrives in damp and humid conditions and can impact various plants. It particularly targets soft fruit that easily rot. Moreover, it can penetrate cracks in woody barks on raspberry and gooseberry stems, leading to die-back. If left untreated, the affected plants need to be completely removed and burned. Other common types of botrytus include B. elliptica which affects Lilly bulbs when it is damp, and B. paeoniae which specifically infects Peonies at the joints where lower leaves meet the stems. This causes brown leaves and mold to appear at ground level. In Europe, stored onions are widely affected by B. allii.
After approximately 10 to 12 weeks in storage, the bulbs become softer and develop brown patches. To prevent this issue, onion ‘sets’ can be treated with dust before planting. B. fabae, which is commonly found on broad beans, causes chocolate brown spots to appear on the leaves, especially on the upper surfaces. This often leads to the patches turning black, and the bean pods becoming infected. As a result, grey mould appears on the pods. In temperate climates, the cold winter weather helps control botrytis by disrupting its life-cycle process and reducing the multiplication of the botrytus fungi.
Most species produce hardened ‘overwintering spores’ as the climate cools. To improve control over fungal outbreaks in future seasons, it is important to prevent the development of these spores along with winter frost. However, when using antibacterial chemicals to combat fungi, it is crucial to bear in mind that repeated use can result in the emergence of mutated and stronger bacterial strains, which may pose problems for humans and other living beings.
The economic impact of botrytis cinerea is significant. This fungus causes damage to grapes in vineyards in France, resulting in a loss of 15-40% of harvests, depending on climatic conditions. This damage accounts for approximately 20% of worldwide harvests in vineyards and is estimated to cost between 10-100 billion Euros annually. Similar losses are also observed in other crops such as strawberries in Spain, where around 20-25% of the crops are affected, and the cut flower industry in Holland, where about 20% of the flowers are impacted. More information can be found on www.genescope.cns.fr about Apple Scab caused by Venturia inaequalis.
Blackish Brown scabby patches appear on fruits, while similar greenish brown spots develop on leaves. The fruits may end up covered with scabby patches and eventually crack or split, becoming vulnerable to secondary problems such as brown rot. Additionally, trees affected by this fungus usually shed their leaves early. The cause of both issues is a closely related fungus that survives the winter on young stems, fallen leaves, and any fruit left on the orchard floor. It is believed that the lack of pruning during damp seasons promotes the spread of this disease due to overcrowded branches.
The suggested controls for Apple Scab, according to Buczacki & Harriss1998, involve improving hygiene by raking up and burning infected leaves to break the fungus’s overwintering cycle. Regular pruning of overcrowded trees is also important, with attention given to removing scabby and swollen shoots. Commercially available anti-fungal sprays like Sovran (kresoxim-methyl) from BASF Company and Flint (trifloxystrobin) from Novartis Company may help combat scab. However, these measures may have limited effect due to the fact that fungal apple scab spores are carried by wind and can spread from tree to tree in orchard situations. (RHS Apple Scab)
Reference Page
Adams C. Bamford K & Early M. 2012 Principles of Horticulture. Oxon UK: Routledge
Alford D 2000, Pest and Disease Management Handbook. Blackwell Science Ltd. Oxford
Baily,A. Chandler, D. Grant, W. Greaves J. Prince G. Tatchell M. 2010 Biopesticides Pest management and Regulation, Oxford: CAB International
Botrytis Reaserch Website – www. Genescope. cns. r http://www. genoscope. cns. fr/spip/Botrytis-cinerea-estimated-losses.html [accessed 18 October 2012]
Buczacki, S. Harris, K. 1998. Pests Diseases & Disorders of Garden Plants, London: Harper Collins
Canadian Phytopathological Society http://phytopath.ca/journallinks.shtml [accessed 27 November]
Flint,M. L and Vanden Bosh, R 1981. Introduction to Integrated Pest Management, New York: Plenum Press
Greenwood P and Halstead A 1997. The royal Horticultural Society. “Pests and Diseases”. The Complete guide to preventing, Identifying & Treating Plant Problems.
The text provides information about the epidemiology of Botrytis cinerea in orchard and vine crops, as well as references to resources on apple scab and wooly aphids. The links to these resources were accessed on 25 September 2012.
