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thegardengeeks
Plant Guide

Diseases of Garlic


Notes:

Introduction

Garlic can be a very easy-to-grow herb in the garden, however it is also prone to several diseases. These include, but are not limited to: Basal Rot (Fusarium culmorum), White Rot (Sclerotium cepivorum),Downy Mildew (Peronospora destructor), Botrytis Rot (Botrytis porri) and Penicillium Decay (Penicillium hirsutum). Most of the major garlic diseases are soilborne, so proper site assessment and yearly rotations are crucial in maintaining a healthy garden of garlic. In addition to these diseases, garlic is also subject to damage by several genera of nematodes.

There are a few physiological disorders of garlic that may alarm the homeowner, but are of generally little consequence to the ability to grow or store garlic. These include: genetic abnormalities and waxy breakdown.

Symptoms

Basal Rot: The symptoms of basal rot are slow to develop. Often, they are seen as a yellowing and eventual dieback of the leaves. Sometimes one can also see white fungal growth at the bulb base, which will lead to both pre and post-harvest rotting. Post-harvest rotting can include single, several or all of the cloves in the garlic bulb.

White Rot: The symptoms of white rot may look almost identical to basal rot, with the exception that the process of disease initiation to plant death is more rapid.  Early symptoms include white, fluffy fungal growth on the stem that extends around the bulb base. Small, dark, over-wintering structures called sclerotia form in the decayed tissue. These can be see in Figure 1 below.

Click to view the larger image
Figure 1: (Microsclerotia of the White Rot fungus Sclerotium cepivorum developing on an infected garlic bulb, provided by the Plant Disease Diagnostic Clinic, Cornell University.)

Downy Mildew: The symptoms of downy mildew are quite distinct: a whitish, furry growth will appear on the leaves, along with yellow discoloration. It can kill younger plants and stunt the growth of older ones.  Diseased leaf tips and other tissues will eventually collapse. Bulbs in storage will have a blackened neck, be shriveled, and outer scales will become water-soaked. Some bulbs may sprout prematurely.

Botrytis Rot: The symptoms of Botrytis Rot include water-soaked stems and gray fuzzy fungal growth. This disease is also called “neckrot.”

Penicillium Decay: Seed clove decay often results in stunted, wilted, and yellowing plants. It can also reduce growth. The fungus may sporulate on diseased cloves, appearing as a bluish-green mass.

Nematodes: There are many symptoms that are associated with specific nematode types. Common symptom themes, however, include erratic plant stand in the field, stunted plants, yellowing, deformed bulbs, and stem swelling.

Physiological Disorders: Genetic abnormalities in garlic can resemble disease symptoms. A common abnormality is variegation of a leaf or entire plant, which can result in reduced photosynthesis or bulb deformation. Waxy breakdown is the degradation of the outer cloves of garlic. Here, sunken tissue will turn a dark yellow color, then become translucent and sticky. Individual cloves will become soft. Unaffected outer scales can obscure these symptoms.

Disease Cycles

Basal Rot: The fungus that causes basal rot prefers high temperatures. It is often considered a weak pathogen, as it will attack plants already damaged by other diseases or insects. Initial infection often occurs through the basal plate. Interestingly, not all infected bulbs show disease symptoms. The pathogen is often spread through fields by infected  seed, through soil and debris transfer from tools etc., and in irrigation water.

White Rot: The fungus that causes white rot prefers cool temperatures, below 75°F. In northern climates, it attacks plants in the spring. The sclerotia can survive in soil for indefinite periods of time in the absence of garlic or other hosts. Sclerotia are stimulated to germinate in the presence of organic sulfur compounds produced by the plants. Once the plants become infected, disease and rot rapidly ensue, either killing the plant outright or causing rot of bulbs in storage.

Downy Mildew: Downy mildews can survive for many years in the soil as oospores. In order to spread and infect, they need to have moist conditions. The spores of downy mildew can swim, so free water is necessary for infection and spread. Additionally, the spores may be spread by the wind.

Botrytis Rot: The fungus will attack garlic plants and bulbs after warm, wet weather. During cooler growing seasons, the disease may not be present in the garden, but may develop on stored bulbs.

Penicillium Decay: Planting infected bulbs spreads Penicillium decay. Infection in the field, however, can occur through the root plate. Average summer high temperatures in upstate NY are ideal for the fungus growth.

Nematodes: Nematodes are soilborne and are often introduced to an area from soil movement due to human activity. Once in the soil, they can usually persist there until there is no longer suitable host tissue for feeding.Ditylenchus dipsaci, Pratylenchus penetrans, and Meloidogyne species are commonly associated with Allium crops.

Physiological Disorders: While physiological disorders do not have disease cycles, there are some common climatic and cultural practices that exasperate them. While the occurrence of genetic abnormalities cannot be clearly defined, it has been noted that waxy breakdown is often the result of high temperatures near harvest.

Management Strategies

If possible, work in clean fields prior to working in fields where infections or infestations have been found. Clean equipment between fields to avoid moving infested soil from one field to another. Additionally, as with any crop, it is important to plant clean healthy seed. For most of the mentioned diseases (Basal Rot, White Rot, Downy Mildew and Nematode infestation), once disease is established, rotation away from Allium crops for several years is an essential management tool.

Basal Rot: Removing infected plants as soon as they are noticed and planting disease-free seed helps manage the disease. In addition, it has been shown that a hot water treatment of the garlic cloves can reduce infection up to 50%.

White Rot: It is advisable to not re-plant in infested fields, but application of some iprodione products (Rovral---an agricultural product---not to be used on residential sites) at planting may help reduce disease incidence. Also avoid planting infested cloves, however pre-treating garlic cloves before planting can help reduce white rot. Hot water pretreatment includes dipping cloves in hot water before planting, though the water should not be above boiling as this will kill the plant.

Downy Mildew: Good air circulation and wide row spacing are important in reducing the occurrence of downy mildew in the field. Several pesticides may be registered to manage or suppress Peronospora spp. on garlic that may be suitable for use in the home garden. Some Serenade formulations and Bonide fung-onil are registered for this use.

Botrytis Rot: Promoting rapid drying at harvest and good aeration in storage is best for managing Botrytis on bulbs. Additionally, cooler storage temperatures may help control the disease. To manage Botrytis Rot in the garden, adequate row spacing and using disease-free bulbs is crucial. Some Serenade formulations are registered to manage or suppress Botrytis spp. on garlic.

Penicillium Decay: Planting cloves as soon as they are cracked and minimizing wounding of the bulbs is the best way to manage Penicillium decay. Quick-drying before storage will also help manage the disease.

Nematodes: Preventing infestation is the best approach. Soil can be tested for the presence of nematodes at a diagnostic facility. Bulbs can also be tested for nematodes before plantings. A hot water treatment may help to kill nematodes in bulbs. Once a field is infested, rotation away from garlic is possibly the only way to effectively reduce nematode populations, although this may not be helpful with genera having broad host ranges.

Physiological Disorders: The physiological disorders caused by genetic abnormalities, in addition to waxy breakdown, cannot be managed in the field. It is important, however, to make sure affected bulbs hold their integrity for storage.

Source:  Cornell University


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